From f6303ce324f401d9873023784b4476bdefb8460c Mon Sep 17 00:00:00 2001
From: Peter Deffebach <p.deffebach@gmail.com>
Date: Sun, 9 Apr 2023 12:57:58 -0400
Subject: [PATCH 01/15] progress

---
 src/macros.jl | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/src/macros.jl b/src/macros.jl
index 51c5a221..f8dd2879 100644
--- a/src/macros.jl
+++ b/src/macros.jl
@@ -1459,11 +1459,17 @@ end
 
 
 function transform_helper(x, args...)
-    x, exprs, outer_flags, kw = get_df_args_kwargs(x, args...; wrap_byrow = false)
+    a1 = first(args)
+    if is_macro_head(a1, "@when")
+        x, exprs, outer_flags, kw = get_df_args_kwargs(x, args[2:end]...; wrap_byrow = false)
+        t = (fun_to_vec(ex; gensym_names = false, outer_flags = outer_flags) for ex in exprs)
 
-    t = (fun_to_vec(ex; gensym_names = false, outer_flags = outer_flags) for ex in exprs)
-    quote
-        $transform($x, $(t...);  $(kw...))
+        z = subset_helper(:($copy($x)), a1.args[2:end]..., :(@kwarg view = true))
+
+        :($parent($transform!($z, $(t...);  $(kw...))))
+    else
+        t = (fun_to_vec(ex; gensym_names = false, outer_flags = outer_flags) for ex in exprs)
+        :($transform($x, $(t...);  $(kw...)))
     end
 end
 

From 8573e3c3e8138cfc8c1cf2b60fabf5751b34c923 Mon Sep 17 00:00:00 2001
From: Peter Deffebach <p.deffebach@gmail.com>
Date: Fri, 15 Dec 2023 14:03:27 -0500
Subject: [PATCH 02/15] toying

---
 src/macros.jl | 30 +++++++++++++++++++++++++++---
 test/when.jl  | 10 ++++++++++
 2 files changed, 37 insertions(+), 3 deletions(-)
 create mode 100644 test/when.jl

diff --git a/src/macros.jl b/src/macros.jl
index dc9b86ae..42df418b 100644
--- a/src/macros.jl
+++ b/src/macros.jl
@@ -1457,17 +1457,41 @@ end
 ##
 ##############################################################################
 
+function contains_when(args...)
+    for arg in args
+        if is_macro_head(arg, "@when")
+            return true
+        end
+    end
+    return false
+end
+
+function when_helper(x, args...)
+    x, exprs, outer_flags, kw = get_df_args_kwargs(x, args[2:end]...; wrap_byrow = false)
+    t = (fun_to_vec(ex; gensym_names = false, outer_flags = outer_flags) for ex in exprs)
+
+    z = subset_helper(:($copy($x)), a1.args[2:end]..., :(@kwarg view = true))
+
+    :($parent($transform!($z, $(t...);  $(kw...))))
+end
 
 function transform_helper(x, args...)
     a1 = first(args)
-    if is_macro_head(a1, "@when")
+    when = is_macro_head(a1, "@when")
+    rwhen = is_macro_head(a1, "@rwhen")
+    if when || rwhen
+        if when
+            z = subset_helper(:($copy($x)), a1.args[2:end]..., :(@kwarg view = true))
+        else #rwhen
+            z = rsubset_helper(:($copy($x)), a1.args[2:end]..., :(@kwarg view = true))
+        end
+
         x, exprs, outer_flags, kw = get_df_args_kwargs(x, args[2:end]...; wrap_byrow = false)
         t = (fun_to_vec(ex; gensym_names = false, outer_flags = outer_flags) for ex in exprs)
 
-        z = subset_helper(:($copy($x)), a1.args[2:end]..., :(@kwarg view = true))
-
         :($parent($transform!($z, $(t...);  $(kw...))))
     else
+        x, exprs, outer_flags, kw = get_df_args_kwargs(x, args...; wrap_byrow = false)
         t = (fun_to_vec(ex; gensym_names = false, outer_flags = outer_flags) for ex in exprs)
         :($transform($x, $(t...);  $(kw...)))
     end
diff --git a/test/when.jl b/test/when.jl
new file mode 100644
index 00000000..04f69554
--- /dev/null
+++ b/test/when.jl
@@ -0,0 +1,10 @@
+module TestWhen
+
+using Test
+using DataFrames
+using DataFramesMeta
+using Statistics
+
+const ≅ = isequal
+
+end # module
\ No newline at end of file

From 2f5f49a3d2f60e8ef1e75a1eae054de1ac5d61c2 Mon Sep 17 00:00:00 2001
From: Peter Deffebach <p.deffebach@gmail.com>
Date: Tue, 19 Dec 2023 09:55:46 -0500
Subject: [PATCH 03/15] implementation

---
 src/macros.jl  | 93 ++++++++++++++++++++++++++++----------------------
 src/parsing.jl |  3 +-
 2 files changed, 54 insertions(+), 42 deletions(-)

diff --git a/src/macros.jl b/src/macros.jl
index 42df418b..63f62c19 100644
--- a/src/macros.jl
+++ b/src/macros.jl
@@ -1456,45 +1456,10 @@ end
 ## transform & @transform
 ##
 ##############################################################################
-
-function contains_when(args...)
-    for arg in args
-        if is_macro_head(arg, "@when")
-            return true
-        end
-    end
-    return false
-end
-
-function when_helper(x, args...)
-    x, exprs, outer_flags, kw = get_df_args_kwargs(x, args[2:end]...; wrap_byrow = false)
-    t = (fun_to_vec(ex; gensym_names = false, outer_flags = outer_flags) for ex in exprs)
-
-    z = subset_helper(:($copy($x)), a1.args[2:end]..., :(@kwarg view = true))
-
-    :($parent($transform!($z, $(t...);  $(kw...))))
-end
-
 function transform_helper(x, args...)
-    a1 = first(args)
-    when = is_macro_head(a1, "@when")
-    rwhen = is_macro_head(a1, "@rwhen")
-    if when || rwhen
-        if when
-            z = subset_helper(:($copy($x)), a1.args[2:end]..., :(@kwarg view = true))
-        else #rwhen
-            z = rsubset_helper(:($copy($x)), a1.args[2:end]..., :(@kwarg view = true))
-        end
-
-        x, exprs, outer_flags, kw = get_df_args_kwargs(x, args[2:end]...; wrap_byrow = false)
-        t = (fun_to_vec(ex; gensym_names = false, outer_flags = outer_flags) for ex in exprs)
-
-        :($parent($transform!($z, $(t...);  $(kw...))))
-    else
-        x, exprs, outer_flags, kw = get_df_args_kwargs(x, args...; wrap_byrow = false)
-        t = (fun_to_vec(ex; gensym_names = false, outer_flags = outer_flags) for ex in exprs)
-        :($transform($x, $(t...);  $(kw...)))
-    end
+    x, exprs, outer_flags, kw = get_df_args_kwargs(x, args...; wrap_byrow = false)
+    t = (fun_to_vec(ex; gensym_names = false, outer_flags = outer_flags) for ex in exprs)
+    :($transform($x, $(t...);  $(kw...)))
 end
 
 """
@@ -1622,12 +1587,58 @@ macro transform(x, args...)
     esc(transform_helper(x, args...))
 end
 
+omit_nested_when(ex, when = Ref(false)) = ex, when
+function omit_nested_when(ex::Expr, when = Ref(false))
+    if ex.head == :macrocall && ex.args[1] in keys(DEFAULT_FLAGS)
+        macroname = ex.args[1]
+        if macroname == WHEN_SYM
+            when[] = true
+            return omit_nested_when(MacroTools.unblock(ex.args[3]), when)
+        else
+            new_expr, when = omit_nested_when(MacroTools.unblock(ex.args[3]), when)
+            ex.args[3] = new_expr
+        end
+    end
+    return ex, when
+end
+
+function get_when_statements(exprs)
+    new_exprs = []
+    when_statements = []
+    seen_non_when = false
+    for expr in exprs
+        e, when = omit_nested_when(expr)
+        if when[]
+            if seen_non_when
+                throw(ArgumentError("All @when statements must come first"))
+            end
+            push!(when_statements, e)
+        else
+            seen_non_when = true
+            push!(new_exprs, expr)
+        end
+    end
+
+    new_exprs, when_statements
+end
+
 function rtransform_helper(x, args...)
     x, exprs, outer_flags, kw = get_df_args_kwargs(x, args...; wrap_byrow = true)
 
-    t = (fun_to_vec(ex; gensym_names=false, outer_flags=outer_flags) for ex in exprs)
-    quote
-        $transform($x, $(t...); $(kw...))
+    exprs, whens = get_when_statements(exprs)
+    if !isempty(whens)
+        w = (fun_to_vec(ex; no_dest = true, gensym_names=false, outer_flags=outer_flags) for ex in whens)
+        t = (fun_to_vec(ex; gensym_names=false, outer_flags=outer_flags) for ex in exprs)
+        z = gensym()
+        quote
+            $z = $subset($copy($x), $(w...); view = true)
+            $parent($transform!($z, $(t...); $(kw...)))
+        end
+    else
+        t = (fun_to_vec(ex; gensym_names=false, outer_flags=outer_flags) for ex in exprs)
+        quote
+            $transform($x, $(t...); $(kw...))
+        end
     end
 end
 
diff --git a/src/parsing.jl b/src/parsing.jl
index 4f1b9e00..99606f78 100644
--- a/src/parsing.jl
+++ b/src/parsing.jl
@@ -167,7 +167,8 @@ is_macro_head(ex::Expr, name) = ex.head == :macrocall && ex.args[1] == Symbol(na
 const BYROW_SYM = Symbol("@byrow")
 const PASSMISSING_SYM = Symbol("@passmissing")
 const ASTABLE_SYM = Symbol("@astable")
-const DEFAULT_FLAGS = (;BYROW_SYM => Ref(false), PASSMISSING_SYM => Ref(false), ASTABLE_SYM => Ref(false))
+const WHEN_SYM = Symbol("@when")
+const DEFAULT_FLAGS = (;BYROW_SYM => Ref(false), PASSMISSING_SYM => Ref(false), ASTABLE_SYM => Ref(false), WHEN_SYM => Ref(false))
 
 extract_macro_flags(ex, exprflags = deepcopy(DEFAULT_FLAGS)) = (ex, exprflags)
 function extract_macro_flags(ex::Expr, exprflags = deepcopy(DEFAULT_FLAGS))

From e7c1a01bcea9f563a7641d1205dacb2493b9c9b2 Mon Sep 17 00:00:00 2001
From: Peter Deffebach <p.deffebach@gmail.com>
Date: Tue, 19 Dec 2023 10:25:26 -0500
Subject: [PATCH 04/15] simplify implementation

---
 src/macros.jl | 28 ++++++++++++++++++++++------
 1 file changed, 22 insertions(+), 6 deletions(-)

diff --git a/src/macros.jl b/src/macros.jl
index 63f62c19..26ea3b15 100644
--- a/src/macros.jl
+++ b/src/macros.jl
@@ -1622,26 +1622,42 @@ function get_when_statements(exprs)
     new_exprs, when_statements
 end
 
-function rtransform_helper(x, args...)
-    x, exprs, outer_flags, kw = get_df_args_kwargs(x, args...; wrap_byrow = true)
+function generic_transform_helper(x, args...; wrap_byrow::Bool = false, modify::Bool = false)
+    if modify == true
+        transformfun = transform!
+    else
+        transformfun = transform
+    end
+    x, exprs, outer_flags, kw = get_df_args_kwargs(x, args...; wrap_byrow = wrap_byrow)
 
     exprs, whens = get_when_statements(exprs)
     if !isempty(whens)
         w = (fun_to_vec(ex; no_dest = true, gensym_names=false, outer_flags=outer_flags) for ex in whens)
         t = (fun_to_vec(ex; gensym_names=false, outer_flags=outer_flags) for ex in exprs)
         z = gensym()
-        quote
-            $z = $subset($copy($x), $(w...); view = true)
-            $parent($transform!($z, $(t...); $(kw...)))
+        if modify
+            quote
+                $z = $subset($x, $(w...); view = true)
+                $parent($transform!($z, $(t...); $(kw...)))
+            end
+        else
+            quote
+                $z = $subset($copy($x), $(w...); view = true)
+                $parent($transform!($z, $(t...); $(kw...)))
+            end
         end
     else
         t = (fun_to_vec(ex; gensym_names=false, outer_flags=outer_flags) for ex in exprs)
         quote
-            $transform($x, $(t...); $(kw...))
+            $transformfun($x, $(t...); $(kw...))
         end
     end
 end
 
+function rtransform_helper(x, args...)
+    generic_transform_helper(x, args...; wrap_byrow = true, modify = false)
+end
+
 """
     @rtransform(x, args...; kwargs...)
 

From 54a497f7f50998c66d67b5942898f855ba7d271b Mon Sep 17 00:00:00 2001
From: Peter Deffebach <p.deffebach@gmail.com>
Date: Tue, 19 Dec 2023 12:33:00 -0500
Subject: [PATCH 05/15] add for with

---
 src/macros.jl  | 175 +++++++++++++++++++------------------------------
 src/parsing.jl |  44 ++++++++++++-
 2 files changed, 109 insertions(+), 110 deletions(-)

diff --git a/src/macros.jl b/src/macros.jl
index 26ea3b15..931dff8d 100644
--- a/src/macros.jl
+++ b/src/macros.jl
@@ -547,11 +547,24 @@ getsinglecolumn(df, s) = throw(ArgumentError("Only indexing with Symbols, string
     "is currently allowed with $DOLLAR"))
 
 function with_helper(d, body)
+    body, outer_flags = extract_macro_flags(body)
+    if body isa Expr && body.head == :block
+        es, whens = get_when_statements(MacroTools.rmlines(body).args)
+    end
     # Make body an expression to force the
     # complicated method of fun_to_vec
     # in the case of QuoteNode
-    t = fun_to_vec(Expr(:block, body); no_dest=true)
-    :(DataFramesMeta.exec($d, $t))
+    t = fun_to_vec(Expr(:block, es...); no_dest=true, outer_flags = outer_flags)
+    if !isempty(whens)
+        w = (fun_to_vec(ex; no_dest = true, gensym_names=false, outer_flags = outer_flags) for ex in whens)
+        z = gensym()
+        quote
+            $z = $subset($d, $(w...); view = true)
+            $exec($z, $t)
+        end
+    else
+        :($exec($d, $t))
+    end
 end
 
 """
@@ -1456,10 +1469,51 @@ end
 ## transform & @transform
 ##
 ##############################################################################
+function generic_transform_select_helper(x, args...; wrap_byrow::Bool = false, modify::Bool = false, selectfun::Bool = false)
+    if selectfun
+        secondstagefun = select!
+        if modify
+            transformfun = select!
+        else
+            transformfun = select
+        end
+    else
+        secondstagefun = transform!
+        if modify
+            transformfun = transform!
+        else
+            transformfun = transform
+        end
+    end
+
+    x, exprs, outer_flags, kw = get_df_args_kwargs(x, args...; wrap_byrow = wrap_byrow)
+
+    exprs, whens = get_when_statements(exprs)
+    if !isempty(whens)
+        w = (fun_to_vec(ex; no_dest = true, gensym_names=false, outer_flags=outer_flags) for ex in whens)
+        t = (fun_to_vec(ex; gensym_names=false, outer_flags=outer_flags) for ex in exprs)
+        z = gensym()
+        if modify
+            quote
+                $z = $subset($x, $(w...); view = true)
+                $parent($secondstagefun($z, $(t...); $(kw...)))
+            end
+        else
+            quote
+                $z = $subset($copy($x), $(w...); view = true)
+                $parent($secondstagefun($z, $(t...); $(kw...)))
+            end
+        end
+    else
+        t = (fun_to_vec(ex; gensym_names=false, outer_flags=outer_flags) for ex in exprs)
+        quote
+            $transformfun($x, $(t...); $(kw...))
+        end
+    end
+end
+
 function transform_helper(x, args...)
-    x, exprs, outer_flags, kw = get_df_args_kwargs(x, args...; wrap_byrow = false)
-    t = (fun_to_vec(ex; gensym_names = false, outer_flags = outer_flags) for ex in exprs)
-    :($transform($x, $(t...);  $(kw...)))
+    generic_transform_select_helper(x, args...; wrap_byrow = false, modify = false)
 end
 
 """
@@ -1587,75 +1641,8 @@ macro transform(x, args...)
     esc(transform_helper(x, args...))
 end
 
-omit_nested_when(ex, when = Ref(false)) = ex, when
-function omit_nested_when(ex::Expr, when = Ref(false))
-    if ex.head == :macrocall && ex.args[1] in keys(DEFAULT_FLAGS)
-        macroname = ex.args[1]
-        if macroname == WHEN_SYM
-            when[] = true
-            return omit_nested_when(MacroTools.unblock(ex.args[3]), when)
-        else
-            new_expr, when = omit_nested_when(MacroTools.unblock(ex.args[3]), when)
-            ex.args[3] = new_expr
-        end
-    end
-    return ex, when
-end
-
-function get_when_statements(exprs)
-    new_exprs = []
-    when_statements = []
-    seen_non_when = false
-    for expr in exprs
-        e, when = omit_nested_when(expr)
-        if when[]
-            if seen_non_when
-                throw(ArgumentError("All @when statements must come first"))
-            end
-            push!(when_statements, e)
-        else
-            seen_non_when = true
-            push!(new_exprs, expr)
-        end
-    end
-
-    new_exprs, when_statements
-end
-
-function generic_transform_helper(x, args...; wrap_byrow::Bool = false, modify::Bool = false)
-    if modify == true
-        transformfun = transform!
-    else
-        transformfun = transform
-    end
-    x, exprs, outer_flags, kw = get_df_args_kwargs(x, args...; wrap_byrow = wrap_byrow)
-
-    exprs, whens = get_when_statements(exprs)
-    if !isempty(whens)
-        w = (fun_to_vec(ex; no_dest = true, gensym_names=false, outer_flags=outer_flags) for ex in whens)
-        t = (fun_to_vec(ex; gensym_names=false, outer_flags=outer_flags) for ex in exprs)
-        z = gensym()
-        if modify
-            quote
-                $z = $subset($x, $(w...); view = true)
-                $parent($transform!($z, $(t...); $(kw...)))
-            end
-        else
-            quote
-                $z = $subset($copy($x), $(w...); view = true)
-                $parent($transform!($z, $(t...); $(kw...)))
-            end
-        end
-    else
-        t = (fun_to_vec(ex; gensym_names=false, outer_flags=outer_flags) for ex in exprs)
-        quote
-            $transformfun($x, $(t...); $(kw...))
-        end
-    end
-end
-
 function rtransform_helper(x, args...)
-    generic_transform_helper(x, args...; wrap_byrow = true, modify = false)
+    generic_transform_select_helper(x, args...; wrap_byrow = true, modify = false)
 end
 
 """
@@ -1703,12 +1690,7 @@ end
 
 
 function transform!_helper(x, args...)
-    x, exprs, outer_flags, kw = get_df_args_kwargs(x, args...; wrap_byrow = false)
-
-    t = (fun_to_vec(ex; gensym_names = false, outer_flags = outer_flags) for ex in exprs)
-    quote
-        $transform!($x, $(t...); $(kw...))
-    end
+    generic_transform_select_helper(x, args...; wrap_byrow = false, modify = true)
 end
 
 """
@@ -1817,12 +1799,7 @@ macro transform!(x, args...)
 end
 
 function rtransform!_helper(x, args...)
-    x, exprs, outer_flags, kw = get_df_args_kwargs(x, args...; wrap_byrow = true)
-
-    t = (fun_to_vec(ex; gensym_names=false, outer_flags=outer_flags) for ex in exprs)
-    quote
-        $transform!($x, $(t...); $(kw...))
-    end
+    generic_transform_select_helper(x, args...; wrap_byrow = true, modify = true)
 end
 
 """
@@ -1841,12 +1818,7 @@ end
 ##############################################################################
 
 function select_helper(x, args...)
-    x, exprs, outer_flags, kw = get_df_args_kwargs(x, args...; wrap_byrow = false)
-
-    t = (fun_to_vec(ex; gensym_names = false, outer_flags = outer_flags) for ex in exprs)
-    quote
-        $select($x, $(t...); $(kw...))
-    end
+    generic_transform_select_helper(x, args...; wrap_byrow = false, modify = false, selectfun = true)
 end
 
 """
@@ -1974,12 +1946,7 @@ macro select(x, args...)
 end
 
 function rselect_helper(x, args...)
-    x, exprs, outer_flags, kw = get_df_args_kwargs(x, args...; wrap_byrow = true)
-
-    t = (fun_to_vec(ex; gensym_names=false, outer_flags=outer_flags) for ex in exprs)
-    quote
-        $select($x, $(t...); $(kw...))
-    end
+    generic_transform_select_helper(x, args...; wrap_byrow = true, modify = false, selectfun = true)
 end
 
 """
@@ -2027,12 +1994,7 @@ end
 ##############################################################################
 
 function select!_helper(x, args...)
-    x, exprs, outer_flags, kw = get_df_args_kwargs(x, args...; wrap_byrow = false)
-
-    t = (fun_to_vec(ex; gensym_names = false, outer_flags = outer_flags) for ex in exprs)
-    quote
-        $select!($x, $(t...); $(kw...))
-    end
+    generic_transform_select_helper(x, args...; wrap_byrow = true, modify = true, selectfun = true)
 end
 
 """
@@ -2155,12 +2117,7 @@ macro select!(x, args...)
 end
 
 function rselect!_helper(x, args...)
-    x, exprs, outer_flags, kw = get_df_args_kwargs(x, args...; wrap_byrow = true)
-
-    t = (fun_to_vec(ex; gensym_names=false, outer_flags=outer_flags) for ex in exprs)
-    quote
-        $select!($x, $(t...); $(kw...))
-    end
+    generic_transform_select_helper(x, args...; wrap_byrow = true, modify = true, selectfun = true)
 end
 
 """
diff --git a/src/parsing.jl b/src/parsing.jl
index 99606f78..2b9a1fd9 100644
--- a/src/parsing.jl
+++ b/src/parsing.jl
@@ -168,7 +168,7 @@ const BYROW_SYM = Symbol("@byrow")
 const PASSMISSING_SYM = Symbol("@passmissing")
 const ASTABLE_SYM = Symbol("@astable")
 const WHEN_SYM = Symbol("@when")
-const DEFAULT_FLAGS = (;BYROW_SYM => Ref(false), PASSMISSING_SYM => Ref(false), ASTABLE_SYM => Ref(false), WHEN_SYM => Ref(false))
+const DEFAULT_FLAGS = (;BYROW_SYM => Ref(false), PASSMISSING_SYM => Ref(false), ASTABLE_SYM => Ref(false))
 
 extract_macro_flags(ex, exprflags = deepcopy(DEFAULT_FLAGS)) = (ex, exprflags)
 function extract_macro_flags(ex::Expr, exprflags = deepcopy(DEFAULT_FLAGS))
@@ -188,6 +188,48 @@ function extract_macro_flags(ex::Expr, exprflags = deepcopy(DEFAULT_FLAGS))
     return (ex, exprflags)
 end
 
+"""
+    omit_nested_when(ex::Expr, when = Ref(false))
+
+For a statement of the form `@passmissing @when x` return `@passmissing x` and
+a flag signifying a `@when` statement was present.
+"""
+function omit_nested_when(ex::Expr, when = Ref(false))
+    if ex.head == :macrocall && ex.args[1] in keys(DEFAULT_FLAGS) || is_macro_head(ex, "@when")
+        macroname = ex.args[1]
+        if macroname == Symbol("@when")
+            when[] = true
+            return omit_nested_when(MacroTools.unblock(ex.args[3]), when)
+        else
+            new_expr, when = omit_nested_when(MacroTools.unblock(ex.args[3]), when)
+            ex.args[3] = new_expr
+        end
+    end
+    return ex, when
+end
+omit_nested_when(ex, when = Ref(false)) = ex, when
+
+function get_when_statements(exprs)
+    new_exprs = []
+    when_statements = []
+    seen_non_when = false
+    for expr in exprs
+        e, when = omit_nested_when(expr)
+        if when[]
+            if seen_non_when
+                throw(ArgumentError("All @when statements must come first"))
+            end
+            push!(when_statements, e)
+        else
+            seen_non_when = true
+            push!(new_exprs, expr)
+        end
+    end
+
+    new_exprs, when_statements
+end
+
+
 """
     check_macro_flags_consistency(exprflags)
 

From d9e45674467e50f607af93c00216760599071b77 Mon Sep 17 00:00:00 2001
From: Peter Deffebach <p.deffebach@gmail.com>
Date: Tue, 19 Dec 2023 14:50:09 -0500
Subject: [PATCH 06/15] add with

---
 src/macros.jl  | 16 ++++++++++++----
 src/parsing.jl |  3 +++
 2 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/src/macros.jl b/src/macros.jl
index 931dff8d..b56df4fa 100644
--- a/src/macros.jl
+++ b/src/macros.jl
@@ -547,14 +547,23 @@ getsinglecolumn(df, s) = throw(ArgumentError("Only indexing with Symbols, string
     "is currently allowed with $DOLLAR"))
 
 function with_helper(d, body)
+    # Get rid of the leading @byrow, @passmissing etc.
+    # but otherwise leave body untouched
     body, outer_flags = extract_macro_flags(body)
-    if body isa Expr && body.head == :block
-        es, whens = get_when_statements(MacroTools.rmlines(body).args)
+    if outer_flags[ASTABLE_SYM][]
+        throw(ArgumentError("@astable macro-flag cannot be used inside of @with"))
     end
+    # If we have a begin...end somewhere, we might
+    # have a @when.
+    # Remove the @when statements, recording that they
+    # exist. To do this we also have to de-construct
+    # body into a vector expressions.
+    es, whens = get_when_statements(MacroTools.rmlines(MacroTools.block(body)).args)
+    newbody = MacroTools.block(es...)
     # Make body an expression to force the
     # complicated method of fun_to_vec
     # in the case of QuoteNode
-    t = fun_to_vec(Expr(:block, es...); no_dest=true, outer_flags = outer_flags)
+    t = fun_to_vec(newbody; no_dest=true, outer_flags = outer_flags)
     if !isempty(whens)
         w = (fun_to_vec(ex; no_dest = true, gensym_names=false, outer_flags = outer_flags) for ex in whens)
         z = gensym()
@@ -1487,7 +1496,6 @@ function generic_transform_select_helper(x, args...; wrap_byrow::Bool = false, m
     end
 
     x, exprs, outer_flags, kw = get_df_args_kwargs(x, args...; wrap_byrow = wrap_byrow)
-
     exprs, whens = get_when_statements(exprs)
     if !isempty(whens)
         w = (fun_to_vec(ex; no_dest = true, gensym_names=false, outer_flags=outer_flags) for ex in whens)
diff --git a/src/parsing.jl b/src/parsing.jl
index 2b9a1fd9..ec436488 100644
--- a/src/parsing.jl
+++ b/src/parsing.jl
@@ -180,6 +180,9 @@ function extract_macro_flags(ex::Expr, exprflags = deepcopy(DEFAULT_FLAGS))
                 throw(ArgumentError("Redundant flag $macroname used."))
             end
             exprflag[] = true
+            if length(ex.args) > 3
+                throw(ArgumentError("Too many arguments passed to $macroname"))
+            end
             return extract_macro_flags(MacroTools.unblock(ex.args[3]), exprflags)
         else
             return (ex, exprflags)

From dc165565275a87762601064e56f86ee24a1d3794 Mon Sep 17 00:00:00 2001
From: Peter Deffebach <p.deffebach@gmail.com>
Date: Tue, 19 Dec 2023 17:22:07 -0500
Subject: [PATCH 07/15] tests

---
 src/macros.jl  |  23 +--
 src/parsing.jl |  14 +-
 test/when.jl   | 435 +++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 458 insertions(+), 14 deletions(-)

diff --git a/src/macros.jl b/src/macros.jl
index b56df4fa..860d7b1f 100644
--- a/src/macros.jl
+++ b/src/macros.jl
@@ -558,17 +558,17 @@ function with_helper(d, body)
     # Remove the @when statements, recording that they
     # exist. To do this we also have to de-construct
     # body into a vector expressions.
-    es, whens = get_when_statements(MacroTools.rmlines(MacroTools.block(body)).args)
-    newbody = MacroTools.block(es...)
+    es, when = get_when_statements(MacroTools.rmlines(MacroTools.block(body)).args)
+    newbody = Expr(:block, es...)
     # Make body an expression to force the
     # complicated method of fun_to_vec
     # in the case of QuoteNode
     t = fun_to_vec(newbody; no_dest=true, outer_flags = outer_flags)
-    if !isempty(whens)
-        w = (fun_to_vec(ex; no_dest = true, gensym_names=false, outer_flags = outer_flags) for ex in whens)
+    if !isnothing(when)
+        w = fun_to_vec(when; no_dest = true, gensym_names=false, outer_flags = outer_flags)
         z = gensym()
         quote
-            $z = $subset($d, $(w...); view = true)
+            $z = $subset($d, $w; view = true, skipmissing = true)
             $exec($z, $t)
         end
     else
@@ -1496,19 +1496,20 @@ function generic_transform_select_helper(x, args...; wrap_byrow::Bool = false, m
     end
 
     x, exprs, outer_flags, kw = get_df_args_kwargs(x, args...; wrap_byrow = wrap_byrow)
-    exprs, whens = get_when_statements(exprs)
-    if !isempty(whens)
-        w = (fun_to_vec(ex; no_dest = true, gensym_names=false, outer_flags=outer_flags) for ex in whens)
+    exprs, when = get_when_statements(exprs)
+   # Main.@infiltrate
+    if !isnothing(when)
+        w = fun_to_vec(when; no_dest = true, gensym_names=false, outer_flags=outer_flags)
         t = (fun_to_vec(ex; gensym_names=false, outer_flags=outer_flags) for ex in exprs)
         z = gensym()
         if modify
             quote
-                $z = $subset($x, $(w...); view = true)
+                $z = $subset($x, $w; view = true, skipmissing = true)
                 $parent($secondstagefun($z, $(t...); $(kw...)))
             end
         else
             quote
-                $z = $subset($copy($x), $(w...); view = true)
+                $z = $subset($copy($x), $w; view = true, skipmissing = true)
                 $parent($secondstagefun($z, $(t...); $(kw...)))
             end
         end
@@ -2002,7 +2003,7 @@ end
 ##############################################################################
 
 function select!_helper(x, args...)
-    generic_transform_select_helper(x, args...; wrap_byrow = true, modify = true, selectfun = true)
+    generic_transform_select_helper(x, args...; wrap_byrow = false, modify = true, selectfun = true)
 end
 
 """
diff --git a/src/parsing.jl b/src/parsing.jl
index ec436488..8cddf510 100644
--- a/src/parsing.jl
+++ b/src/parsing.jl
@@ -200,6 +200,9 @@ a flag signifying a `@when` statement was present.
 function omit_nested_when(ex::Expr, when = Ref(false))
     if ex.head == :macrocall && ex.args[1] in keys(DEFAULT_FLAGS) || is_macro_head(ex, "@when")
         macroname = ex.args[1]
+        if length(ex.args) > 3
+            throw(ArgumentError("Too many arguments passed to $macroname"))
+        end
         if macroname == Symbol("@when")
             when[] = true
             return omit_nested_when(MacroTools.unblock(ex.args[3]), when)
@@ -214,22 +217,27 @@ omit_nested_when(ex, when = Ref(false)) = ex, when
 
 function get_when_statements(exprs)
     new_exprs = []
-    when_statements = []
+    when_statement = nothing
     seen_non_when = false
+    seen_when = false
     for expr in exprs
         e, when = omit_nested_when(expr)
         if when[]
+            if seen_when
+                throw(ArgumentError("Only one @when statement allowed at a time"))
+            end
             if seen_non_when
                 throw(ArgumentError("All @when statements must come first"))
             end
-            push!(when_statements, e)
+            seen_when = true
+            when_statement = e
         else
             seen_non_when = true
             push!(new_exprs, expr)
         end
     end
 
-    new_exprs, when_statements
+    new_exprs, when_statement
 end
 
 
diff --git a/test/when.jl b/test/when.jl
index 04f69554..083e19c0 100644
--- a/test/when.jl
+++ b/test/when.jl
@@ -7,4 +7,439 @@ using Statistics
 
 const ≅ = isequal
 
+@testset "@transform when" begin
+    df = DataFrame(a = [1, 2], z = [60, 70])
+    res = DataFrame(a = [1, 2], z = [60, 500], c = [missing, 5])
+    df2 = @transform df begin
+        @when :a .> 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+
+    df2 = @transform(df, @when(:a .> 1), :c = 5, :z = 500)
+    @test df2 ≅ res
+
+    df2 = @transform df @byrow begin
+        @when :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+
+    df2 = @transform df @byrow @passmissing begin
+        @when :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+
+    df2 = @transform df begin
+        @byrow @when :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+
+    df2 = @transform df begin
+        @when @byrow :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+
+
+    df2 = @transform df begin
+        @when @byrow :a > 1 ? true : missing
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+
+    dfa = copy(df)
+    dfa.a = [missing, 2]
+    df2 = @transform dfa begin
+        @when @passmissing @byrow :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ DataFrame(a = [missing, 2], z = [60, 500], c = [missing, 5])
+end
+
+@testset "@rtransform when" begin
+    df = DataFrame(a = [1, 2], z = [60, 70])
+    res = DataFrame(a = [1, 2], z = [60, 500], c = [missing, 5])
+    df2 = @rtransform df begin
+        @when :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+
+    df2 = @rtransform(df, @when(:a > 1), :c = 5, :z = 500)
+    @test df2 ≅ res
+
+    df2 = @rtransform df @passmissing begin
+        @when :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+
+    df2 = @rtransform df begin
+        @when :a > 1 ? true : missing
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+
+    dfa = copy(df)
+    dfa.a = [missing, 2]
+    df2 = @transform dfa begin
+        @when @passmissing @byrow :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ DataFrame(a = [missing, 2], z = [60, 500], c = [missing, 5])
+end
+
+@testset "@transform! when" begin
+    df_orig = DataFrame(a = [1, 2], z = [60, 70])
+    res = DataFrame(a = [1, 2], z = [60, 500], c = [missing, 5])
+    df = copy(df_orig)
+    df2 = @transform! df begin
+        @when :a .> 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+    @test df2 === df
+
+    df = copy(df_orig)
+    df2 = @transform!(df, @when(:a .> 1), :c = 5, :z = 500)
+    @test df2 ≅ res
+
+    df = copy(df_orig)
+    df2 = @transform! df @byrow begin
+        @when :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+    @test df2 === df
+
+    df = copy(df_orig)
+    df2 = @transform! df @byrow @passmissing begin
+        @when :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+    @test df2 === df
+
+    df = copy(df_orig)
+    df2 = @transform! df begin
+        @byrow @when :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+    @test df2 === df
+
+    df = copy(df_orig)
+    df2 = @transform! df begin
+        @when @byrow :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+    @test df2 === df
+
+    df = copy(df_orig)
+    df2 = @transform! df begin
+        @when @byrow :a > 1 ? true : missing
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+    @test df2 === df
+
+    dfa = copy(df_orig)
+    dfa.a = [missing, 2]
+    df = copy(dfa)
+    df2 = @transform! df begin
+        @when @passmissing @byrow :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ DataFrame(a = [missing, 2], z = [60, 500], c = [missing, 5])
+    @test df2 === df
+end
+
+
+@testset "@rtransform! when" begin
+    df_orig = DataFrame(a = [1, 2], z = [60, 70])
+    res = DataFrame(a = [1, 2], z = [60, 500], c = [missing, 5])
+    df = copy(df_orig)
+    df2 = @rtransform! df begin
+        @when :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+    @test df2 === df
+
+    df = copy(df_orig)
+    df2 = @rtransform!(df, @when(:a > 1), :c = 5, :z = 500)
+    @test df2 ≅ res
+
+    df = copy(df_orig)
+    df2 = @rtransform! df begin
+        @when :a > 1 ? true : missing
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+    @test df2 === df
+
+    dfa = copy(df_orig)
+    dfa.a = [missing, 2]
+    df = copy(dfa)
+    df2 = @rtransform! df begin
+        @when @passmissing :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ DataFrame(a = [missing, 2], z = [60, 500], c = [missing, 5])
+    @test df2 === df
+end
+
+@testset "@select when" begin
+    df = DataFrame(a = [1, 2], z = [60, 70])
+    res = DataFrame(c = [missing, 5], z = [60, 500])
+    df2 = @select df begin
+        @when :a .> 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+
+    df2 = @select(df, @when(:a .> 1), :c = 5, :z = 500)
+    @test df2 ≅ res
+
+    df2 = @select df @byrow begin
+        @when :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+
+    df2 = @select df @byrow @passmissing begin
+        @when :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+
+    df2 = @select df begin
+        @byrow @when :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+
+    df2 = @select df begin
+        @when @byrow :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+
+
+    df2 = @select df begin
+        @when @byrow :a > 1 ? true : missing
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+
+    dfa = copy(df)
+    dfa.a = [missing, 2]
+    df2 = @select dfa begin
+        @when @passmissing @byrow :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+end
+
+@testset "@rselect when" begin
+    df = DataFrame(a = [1, 2], z = [60, 70])
+    res = DataFrame(c = [missing, 5], z = [60, 500])
+    df2 = @rselect df begin
+        @when :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+
+    df2 = @rselect(df, @when(:a > 1), :c = 5, :z = 500)
+    @test df2 ≅ res
+
+    df2 = @rselect df @passmissing begin
+        @when :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+
+    df2 = @rselect df begin
+        @when :a > 1 ? true : missing
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+
+    dfa = copy(df)
+    dfa.a = [missing, 2]
+    df2 = @select dfa begin
+        @when @passmissing @byrow :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+end
+
+@testset "@select! when" begin
+    df_orig = DataFrame(a = [1, 2], z = [60, 70])
+    res = DataFrame(c = [missing, 5], z = [60, 500])
+    df = copy(df_orig)
+    df2 = @select! df begin
+        @when :a .> 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+    @test df2 === df
+
+    df = copy(df_orig)
+    df2 = @select!(df, @when(:a .> 1), :c = 5, :z = 500)
+    @test df2 ≅ res
+
+    df = copy(df_orig)
+    df2 = @select! df @byrow begin
+        @when :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+    @test df2 === df
+
+    df = copy(df_orig)
+    df2 = @select! df @byrow @passmissing begin
+        @when :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+    @test df2 === df
+
+    df = copy(df_orig)
+    df2 = @select! df begin
+        @byrow @when :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+    @test df2 === df
+
+    df = copy(df_orig)
+    df2 = @select! df begin
+        @when @byrow :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+    @test df2 === df
+
+    df = copy(df_orig)
+    df2 = @select! df begin
+        @when @byrow :a > 1 ? true : missing
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+    @test df2 === df
+
+    dfa = copy(df_orig)
+    dfa.a = [missing, 2]
+    df = copy(dfa)
+    df2 = @select! df begin
+        @when @passmissing @byrow :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+    @test df2 === df
+end
+
+
+@testset "@rselect! when" begin
+    df_orig = DataFrame(a = [1, 2], z = [60, 70])
+    res = DataFrame(c = [missing, 5], z = [60, 500])
+    df = copy(df_orig)
+    df2 = @rselect! df begin
+        @when :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+    @test df2 === df
+
+    df = copy(df_orig)
+    df2 = @rselect!(df, @when(:a > 1), :c = 5, :z = 500)
+    @test df2 ≅ res
+
+    df = copy(df_orig)
+    df2 = @rselect! df begin
+        @when :a > 1 ? true : missing
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+    @test df2 === df
+
+    dfa = copy(df_orig)
+    dfa.a = [missing, 2]
+    df = copy(dfa)
+    df2 = @rselect! df begin
+        @when @passmissing :a > 1
+        :c = 5
+        :z = 500
+    end
+    @test df2 ≅ res
+    @test df2 === df
+end
+
+@testset "@when many conditions" begin
+    df = DataFrame(a = [1, missing, 3, 4], z = [50, 60, 70, 80])
+    @transform df begin
+        @when :a .> 1
+        @when :a .> 2
+        :c = 5
+    end
+
+end
+
+
+@testset "@with when" begin
+    df = DataFrame(a = [1, 2], z = [60, 70])
+
+    t = @with df begin
+        @when :a .> 1
+        :z
+    end
+    @test t === view(df.z, 2:2)
+end
+
 end # module
\ No newline at end of file

From 4bec3c0f66660bc6c0c059c1bf71201a4715143c Mon Sep 17 00:00:00 2001
From: Peter Deffebach <p.deffebach@gmail.com>
Date: Tue, 19 Dec 2023 17:29:15 -0500
Subject: [PATCH 08/15] more tests

---
 test/when.jl | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/test/when.jl b/test/when.jl
index 083e19c0..1a3e238a 100644
--- a/test/when.jl
+++ b/test/when.jl
@@ -421,14 +421,28 @@ end
     @test df2 === df
 end
 
-@testset "@when many conditions" begin
-    df = DataFrame(a = [1, missing, 3, 4], z = [50, 60, 70, 80])
-    @transform df begin
+df = DataFrame(a = [1, missing, 3, 4], z = [50, 60, 70, 80])
+@testset "@when errors" begin
+    @test_throws LoadError @eval @transform df begin
         @when :a .> 1
         @when :a .> 2
         :c = 5
     end
 
+    @test_throws LoadError @eval @transform df @when(:a .== 1) begin
+        :c = 1
+        :b = 2
+    end
+
+    @test_throws LoadError @eval @transform df @byrow @when(:a == 1) begin
+        :c = 1
+        :b = 2
+    end
+
+    @test_throws LoadError @eval @transform df @when(:a == 1) @byrow begin
+        :c = 1
+        :b = 2
+    end
 end
 
 

From 9db9123bfc5bb2d3391e8ca2a0e1e230929f25e9 Mon Sep 17 00:00:00 2001
From: Peter Deffebach <p.deffebach@gmail.com>
Date: Tue, 19 Dec 2023 17:46:20 -0500
Subject: [PATCH 09/15] add tests for with

---
 test/when.jl | 31 +++++++++++++++++++++++++++++--
 1 file changed, 29 insertions(+), 2 deletions(-)

diff --git a/test/when.jl b/test/when.jl
index 1a3e238a..dac0ff4e 100644
--- a/test/when.jl
+++ b/test/when.jl
@@ -443,17 +443,44 @@ df = DataFrame(a = [1, missing, 3, 4], z = [50, 60, 70, 80])
         :c = 1
         :b = 2
     end
-end
+
+    @test_throws LoadError @eval @transform df @astable begin
+        @when :x == 1
+        :z = 1
+    end
 
 
+    @test_throws LoadError @eval @with df @when(:a == 1) begin
+        first(:z)
+    end
+end
+
 @testset "@with when" begin
-    df = DataFrame(a = [1, 2], z = [60, 70])
+    df = DataFrame(a = [missing, 2], z = [60, 70])
 
     t = @with df begin
         @when :a .> 1
         :z
     end
     @test t === view(df.z, 2:2)
+
+    t = @with df @byrow begin
+        @when :a > 1
+        first(:z)
+    end
+    @test t == [70]
+
+    t = @with df begin
+        @when @byrow :a > 1
+        first(:z)
+    end
+    @test t == 70
+
+    t = @with df begin
+        @when @byrow @passmissing :a > 1 && true
+        first(:z)
+    end
+    @test t == 70
 end
 
 end # module
\ No newline at end of file

From 8807032392eedf8d07bb637e2d46c4054fa24a94 Mon Sep 17 00:00:00 2001
From: Peter Deffebach <p.deffebach@gmail.com>
Date: Wed, 20 Dec 2023 10:48:25 -0500
Subject: [PATCH 10/15] progress on docs

---
 docs/src/index.md     | 129 +++++++++++++++++++++++++++++++++++++++++-
 src/DataFramesMeta.jl |   2 +-
 src/macros.jl         |  77 +++++++++++++++++++++++++
 3 files changed, 206 insertions(+), 2 deletions(-)

diff --git a/docs/src/index.md b/docs/src/index.md
index 062aaee2..f8eb2c46 100644
--- a/docs/src/index.md
+++ b/docs/src/index.md
@@ -24,6 +24,7 @@ In addition, DataFramesMeta provides
 * `@byrow` for applying functions to each row of a data frame (only supported inside other macros).
 * `@passmissing` for propagating missing values inside row-wise DataFramesMeta.jl transformations.
 * `@astable` to create multiple columns within a single transformation.
+* `@when` to non-destructively work with a subset of observations (Similar to Stata's `if`)
 * `@chain`, from [Chain.jl](https://github.com/jkrumbiegel/Chain.jl) for piping the above macros together, similar to [magrittr](https://cran.r-project.org/web/packages/magrittr/vignettes/magrittr.html)'s
   `%>%` in R. 
 
@@ -47,7 +48,133 @@ Use `passmissing`  to propagate `missing` values more easily. See `?passmissing`
 details. `passmissing` is defined in [Missings.jl](https://github.com/JuliaData/Missings.jl)
 but exported by DataFramesMeta for convenience. 
 
-# Provided macros
+# Provided macrosp = graph_mean_outp = graph_mean_outcomes_bins(
+  "dev_status", 
+  "income_pctile",  
+  "distance_bin", p = graph_mean_outcomes_bins(
+  "dev_status", 
+  "income_pctile",  
+  "distance_bin", 
+  "hilly", 
+  "pop_weight_country", 
+  long_shapes)
+
+  "hilly", 
+  "pop_weight_country", 
+  long_shapes)
+p = graph_mean_outcomes_bins(
+  "dev_status", 
+  "income_pctile",  
+  "distance_bin", 
+  "hilly", 
+  "pop_weight_country", 
+  long_shapes)
+p = graph_mean_outcomes_bins(
+  "dev_status", 
+  "income_pctile",  
+  "distance_bin", 
+  "hilly", 
+  "pop_weight_country", 
+  long_shapes)
+p = graph_mean_outcomes_bins(
+  "dev_status", 
+  "income_pctile",  
+  "distance_bin", 
+  "hilly", 
+  "pop_weight_country", 
+  long_shapes)
+p = graph_mean_outcomes_bins(
+  "dev_status", 
+  "income_pctile",  
+  "distance_bin", 
+  "hilly", 
+  "pop_weight_country", 
+  long_shapes)
+p = graph_mean_outcomes_bins(
+  "dev_status", 
+  "income_pctile",  
+  "distance_bin", 
+  "hilly", 
+  "pop_weight_country", 
+  long_shapes)
+p = graph_mean_outcomes_bins(
+  "dev_status", 
+  "income_pctile",  
+  "distance_bin", 
+  "hilly", 
+  "pop_weight_country", 
+  long_shapes)
+p = graph_mean_outcomes_bins(
+  "dev_status", 
+  "income_pctile",  
+  "distance_bin", 
+  "hilly", 
+  "pop_weight_country", 
+  long_shapes)
+p = graph_mean_outcomes_bins(
+  "dev_status", 
+  "income_pctile",  
+  "distance_bin", 
+  "hilly", 
+  "pop_weight_country", 
+  long_shapes)
+p = graph_mean_outcomes_bins(
+  "dev_status", 
+  "income_pctile",  
+  "distance_bin", 
+  "hilly", 
+  "pop_weight_country", 
+  long_shapes)
+p = graph_mean_outcomes_bins(
+  "dev_status", 
+  "income_pctile",  
+  "distance_bin", 
+  "hilly", 
+  "pop_weight_country", 
+  long_shapes)
+p = graph_mean_outcomes_bins(
+  "dev_status", 
+  "income_pctile",  
+  "distance_bin", 
+  "hilly", 
+  "pop_weight_country", 
+  long_shapes)
+p = graph_mean_outcomes_bins(
+  "dev_status", 
+  "income_pctile",  
+  "distance_bin", 
+  "hilly", 
+  "pop_weight_country", 
+  long_shapes)
+cp = graph_mean_outcomes_bins(
+  "dev_status", 
+  "income_pctile",  
+  "distance_bin", 
+  "hilly", 
+  "pop_weight_country", 
+  long_shapes)
+p = graph_mean_outcomes_bins(
+  "dev_status", 
+  "income_pctile",  
+  "distance_bin", 
+  "hilly", 
+  "pop_weight_country", 
+  long_shapes)
+op = graph_mean_outcomes_bins(
+  "dev_status", 
+  "income_pctile",  
+  "distance_bin", 
+  "hilly", 
+  "pop_weight_country", 
+  long_shapes)
+mes_bins(
+  "dev_status", 
+  "income_pctile",  
+  "distance_bin", 
+  "hilly", 
+  "pop_weight_country", 
+  long_shapes)
+
 
 !!! note 
     
diff --git a/src/DataFramesMeta.jl b/src/DataFramesMeta.jl
index a16cca5a..a96916d7 100644
--- a/src/DataFramesMeta.jl
+++ b/src/DataFramesMeta.jl
@@ -20,7 +20,7 @@ export @with,
        @rtransform, @rselect, @rtransform!, @rselect!,
        @distinct, @rdistinct, @distinct!, @rdistinct!,
        @eachrow, @eachrow!,
-       @byrow, @passmissing, @astable, @kwarg,
+       @byrow, @passmissing, @astable, @kwarg, @when,
        @based_on, @where # deprecated
 
 const DOLLAR = raw"$"
diff --git a/src/macros.jl b/src/macros.jl
index 860d7b1f..55e2a175 100644
--- a/src/macros.jl
+++ b/src/macros.jl
@@ -685,6 +685,83 @@ macro with(d, body)
     esc(with_helper(d, body))
 end
 
+"""
+    @when(args...)
+
+Perform operations on a subset of `df`, but still
+return a data frame with the same number of rows as `df`. `@when` can be used
+with the `@transform` macros, `@select` macros, and `@with`.
+
+`@when` is not a "real" macro. It is only functional inside DataFramesMeta.jl macros.
+A motivating example:
+
+```
+@rtransform df begin
+    @when :a == 1
+    :y = :y - mean(:y)
+end
+```
+
+The above block generates the column `:y` which is de-meaned with respect to observations where
+`:a == 1`. If `:y` already exists in `df`, then new values over-write old values only
+when `:a == 1`. If `:y` does not already exist in `df`, then new values are written
+when `:a == 1`, and remaining values are filled with `missing`.
+
+Only one `@when` statement is allowed per transformation macro and it must be the
+first argument in the transformation.
+
+`@when` inherits `@byrow` and `@passmissing` from the transformation. As an example:
+
+```
+@transform df @byrow begin
+    @when :a == 1
+    ...
+end
+```
+
+In the above, the condition inside `@when` operates row-wise. However, `@byrow` and `@passmissing` can
+also be passed independently, such as `@byrow @when :a == 1`.
+
+Like `@subset`, `@when` drops rows where `missing` values are returned. Unlike `@subset`,
+there is currently no way to control this behavior.
+
+## Details
+
+`@when` operates by calling `select` with the `view = true` keyword argument,
+followed by a `transform!` call. See `?transform!` for more details. Roughly,
+the expression
+
+```
+@transform df begin
+    @when :a .== 1
+    :y = 5
+end
+```
+
+translates to
+
+```
+df1 = @subset(copy(df), :a .== 1; view = true)
+df2 = @transform! df1 :y = 5
+parent(df2)
+```
+
+Unlike the other macro-flags, such as `@passmissing` and `@byrow`, `@when` cannot be
+used at the top-level.
+```
+@transform df @byrow @when(:a == 1) begin
+    :x = 1
+    :y = 2
+end
+```
+is not supported.
+
+"""
+macro when(args...)
+    throw(ArgumentError("@passmissing only works inside DataFramesMeta macros."))
+end
+
+
 ASTABLE_RHS_ORDERBY_DOCS = """
 In operations, it is also allowed to use `AsTable(cols)` to work with
 multiple columns at once, where the columns are grouped together in a

From 2c0e1c2fda08e29f18b3342854b44c654681fbe7 Mon Sep 17 00:00:00 2001
From: Peter Deffebach <p.deffebach@gmail.com>
Date: Thu, 21 Dec 2023 10:48:17 -0500
Subject: [PATCH 11/15] fix diff

---
 docs/src/index.md | 128 +---------------------------------------------
 1 file changed, 1 insertion(+), 127 deletions(-)

diff --git a/docs/src/index.md b/docs/src/index.md
index f8eb2c46..6b176f79 100644
--- a/docs/src/index.md
+++ b/docs/src/index.md
@@ -48,133 +48,7 @@ Use `passmissing`  to propagate `missing` values more easily. See `?passmissing`
 details. `passmissing` is defined in [Missings.jl](https://github.com/JuliaData/Missings.jl)
 but exported by DataFramesMeta for convenience. 
 
-# Provided macrosp = graph_mean_outp = graph_mean_outcomes_bins(
-  "dev_status", 
-  "income_pctile",  
-  "distance_bin", p = graph_mean_outcomes_bins(
-  "dev_status", 
-  "income_pctile",  
-  "distance_bin", 
-  "hilly", 
-  "pop_weight_country", 
-  long_shapes)
-
-  "hilly", 
-  "pop_weight_country", 
-  long_shapes)
-p = graph_mean_outcomes_bins(
-  "dev_status", 
-  "income_pctile",  
-  "distance_bin", 
-  "hilly", 
-  "pop_weight_country", 
-  long_shapes)
-p = graph_mean_outcomes_bins(
-  "dev_status", 
-  "income_pctile",  
-  "distance_bin", 
-  "hilly", 
-  "pop_weight_country", 
-  long_shapes)
-p = graph_mean_outcomes_bins(
-  "dev_status", 
-  "income_pctile",  
-  "distance_bin", 
-  "hilly", 
-  "pop_weight_country", 
-  long_shapes)
-p = graph_mean_outcomes_bins(
-  "dev_status", 
-  "income_pctile",  
-  "distance_bin", 
-  "hilly", 
-  "pop_weight_country", 
-  long_shapes)
-p = graph_mean_outcomes_bins(
-  "dev_status", 
-  "income_pctile",  
-  "distance_bin", 
-  "hilly", 
-  "pop_weight_country", 
-  long_shapes)
-p = graph_mean_outcomes_bins(
-  "dev_status", 
-  "income_pctile",  
-  "distance_bin", 
-  "hilly", 
-  "pop_weight_country", 
-  long_shapes)
-p = graph_mean_outcomes_bins(
-  "dev_status", 
-  "income_pctile",  
-  "distance_bin", 
-  "hilly", 
-  "pop_weight_country", 
-  long_shapes)
-p = graph_mean_outcomes_bins(
-  "dev_status", 
-  "income_pctile",  
-  "distance_bin", 
-  "hilly", 
-  "pop_weight_country", 
-  long_shapes)
-p = graph_mean_outcomes_bins(
-  "dev_status", 
-  "income_pctile",  
-  "distance_bin", 
-  "hilly", 
-  "pop_weight_country", 
-  long_shapes)
-p = graph_mean_outcomes_bins(
-  "dev_status", 
-  "income_pctile",  
-  "distance_bin", 
-  "hilly", 
-  "pop_weight_country", 
-  long_shapes)
-p = graph_mean_outcomes_bins(
-  "dev_status", 
-  "income_pctile",  
-  "distance_bin", 
-  "hilly", 
-  "pop_weight_country", 
-  long_shapes)
-p = graph_mean_outcomes_bins(
-  "dev_status", 
-  "income_pctile",  
-  "distance_bin", 
-  "hilly", 
-  "pop_weight_country", 
-  long_shapes)
-cp = graph_mean_outcomes_bins(
-  "dev_status", 
-  "income_pctile",  
-  "distance_bin", 
-  "hilly", 
-  "pop_weight_country", 
-  long_shapes)
-p = graph_mean_outcomes_bins(
-  "dev_status", 
-  "income_pctile",  
-  "distance_bin", 
-  "hilly", 
-  "pop_weight_country", 
-  long_shapes)
-op = graph_mean_outcomes_bins(
-  "dev_status", 
-  "income_pctile",  
-  "distance_bin", 
-  "hilly", 
-  "pop_weight_country", 
-  long_shapes)
-mes_bins(
-  "dev_status", 
-  "income_pctile",  
-  "distance_bin", 
-  "hilly", 
-  "pop_weight_country", 
-  long_shapes)
-
+# Provided macros
 
 !!! note 
     

From 58cb68ad94bfac9f028fe2c5d49de21b77981d12 Mon Sep 17 00:00:00 2001
From: Peter Deffebach <p.deffebach@gmail.com>
Date: Thu, 21 Dec 2023 15:52:54 -0500
Subject: [PATCH 12/15] add when.jl to tests

---
 test/runtests.jl | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/test/runtests.jl b/test/runtests.jl
index 3cb43e93..5af4ce06 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -15,7 +15,8 @@ my_tests = ["dataframes.jl",
             "byrow.jl",
             "astable.jl",
             "astable_flag.jl",
-            "passmissing.jl"]
+            "passmissing.jl",
+            "when.jl"]
 
 println("Running tests:")
 

From ff32e7fbcbdf514cdc921453c7a97a70c8aa235f Mon Sep 17 00:00:00 2001
From: Peter Deffebach <p.deffebach@gmail.com>
Date: Thu, 28 Mar 2024 11:50:47 -0400
Subject: [PATCH 13/15] merge in master again

---
 src/DataFramesMeta.jl | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/src/DataFramesMeta.jl b/src/DataFramesMeta.jl
index 4e96c4c9..330f7c6a 100644
--- a/src/DataFramesMeta.jl
+++ b/src/DataFramesMeta.jl
@@ -24,13 +24,9 @@ export @with,
        @rtransform, @rselect, @rtransform!, @rselect!,
        @distinct, @rdistinct, @distinct!, @rdistinct!,
        @eachrow, @eachrow!,
-<<<<<<< HEAD
-       @byrow, @passmissing, @astable, @kwarg, @when,
-=======
-       @byrow, @passmissing, @astable, @kwarg,
+       @byrow, @passmissing, @astable, @kwarg, @when
        @label!, @note!, printlabels, printnotes,
        @groupby,
->>>>>>> master
        @based_on, @where # deprecated
 
 const DOLLAR = raw"$"

From f166cb2ddf0a7a2895930e5179ee030c791f228d Mon Sep 17 00:00:00 2001
From: Peter Deffebach <p.deffebach@gmail.com>
Date: Thu, 28 Mar 2024 12:02:42 -0400
Subject: [PATCH 14/15] more merge fixes

---
 src/DataFramesMeta.jl | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/DataFramesMeta.jl b/src/DataFramesMeta.jl
index 330f7c6a..553b5fa5 100644
--- a/src/DataFramesMeta.jl
+++ b/src/DataFramesMeta.jl
@@ -24,7 +24,7 @@ export @with,
        @rtransform, @rselect, @rtransform!, @rselect!,
        @distinct, @rdistinct, @distinct!, @rdistinct!,
        @eachrow, @eachrow!,
-       @byrow, @passmissing, @astable, @kwarg, @when
+       @byrow, @passmissing, @astable, @kwarg, @when,
        @label!, @note!, printlabels, printnotes,
        @groupby,
        @based_on, @where # deprecated

From 2e1692bedd93896fb4441acaf1d6aa8ffa01da43 Mon Sep 17 00:00:00 2001
From: Peter Deffebach <p.deffebach@gmail.com>
Date: Fri, 29 Mar 2024 10:57:17 -0400
Subject: [PATCH 15/15] handle grouped data frames

---
 src/macros.jl | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/macros.jl b/src/macros.jl
index 409b2041..d3baae33 100644
--- a/src/macros.jl
+++ b/src/macros.jl
@@ -1555,6 +1555,8 @@ end
 ## transform & @transform
 ##
 ##############################################################################
+copy_gd(x::GroupedDataFrame) = transform(x; ungroup = false)
+copy_gd(x::AbstractDataFrame) = copy(x)
 function generic_transform_select_helper(x, args...; wrap_byrow::Bool = false, modify::Bool = false, selectfun::Bool = false)
     if selectfun
         secondstagefun = select!
@@ -1586,7 +1588,7 @@ function generic_transform_select_helper(x, args...; wrap_byrow::Bool = false, m
             end
         else
             quote
-                $z = $subset($copy($x), $w; view = true, skipmissing = true)
+                $z = $subset($copy_gd($x), $w; view = true, skipmissing = true)
                 $parent($secondstagefun($z, $(t...); $(kw...)))
             end
         end