Merge pull request #164 from JuliaLang/jr/withenv

Add withenv

README.md

@@ -58,6 +58,8 @@ Currently, the `@compat` macro supports the following syntaxes:
 
 * `@compat Union{args...}` - `Union(args...)` on 0.3. [#11432](https://github.com/JuliaLang/julia/pull/11432)
 
+* `@compat withenv(f, "a" => a, "b" => b...)` on 0.3.
+
 ## Type Aliases
 
 * `typealias AbstractString String` - `String` has been renamed to `AbstractString` [#8872](https://github.com/JuliaLang/julia/pull/8872)
@@ -99,6 +101,8 @@ Currently, the `@compat` macro supports the following syntaxes:
 
 * `isapprox(A, B)` for arrays ([JuliaLang/julia#12472](https://github.com/JuliaLang/julia/pull/12472)), and synonyms `≈` ([U+2248](http://www.fileformat.info/info/unicode/char/2248/index.htm), LaTeX `\approx`) and `≉` ([U+2249](http://www.fileformat.info/info/unicode/char/2249/index.htm), LaTeX `\napprox`) for `isapprox` and `!isapprox`, respectively.
 
+* `withenv` can be used in julia 0.3 (see [the 0.4 docs](http://docs.julialang.org/en/release-0.4/stdlib/base/#Base.withenv)). Note that you must prepend calls to `withenv` with `@compat` if you'd like to use it with the `=>` syntax.
+
 ## Renamed functions
 
 * `itrunc`, `iround`, `iceil`, `ifloor` are now accessed via `trunc(T, x)`, etc. ([#9133](https://github.com/JuliaLang/julia/pull/9133)).  Truncated conversions between integer types are now `n % T` ([#8646](https://github.com/JuliaLang/julia/issues/8646)).

src/Compat.jl

@@ -279,6 +279,17 @@ function rewrite_split(ex, f)
     end
 end
 
+# rewrites all subexpressions of the form `a => b` to `(a, b)`
+function rewrite_pairs_to_tuples!(expr::Expr)
+    if expr.head == :(=>)
+        expr.head = :tuple
+    end
+    for subexpr in expr.args
+        isa(subexpr, Expr) && rewrite_pairs_to_tuples!(subexpr)
+    end
+    return expr
+end
+
 if VERSION < v"0.4.0-dev+707"
     macro inline(ex)
         esc(ex)
@@ -395,6 +406,8 @@ function _compat(ex::Expr)
             else
                 ex = Expr(:call, :Array, f.args[2], ex.args[2:end]...)
             end
+        elseif VERSION < v"0.4.0-dev+4389" && f == :withenv
+            rewrite_pairs_to_tuples!(ex)
         end
     elseif ex.head == :curly
         f = ex.args[1]
@@ -690,6 +703,26 @@ if VERSION < v"0.4.0-dev+3837"
     const OutOfMemoryError = MemoryError
 end
 
+
+if VERSION < v"0.4.0-dev+4389"
+    export withenv
+    # temporarily set and then restore an environment value
+    function withenv(f::Function, keyvals...)
+        old = Dict{typeof(first(first(keyvals))),Any}()
+        for (key,val) in keyvals
+            old[key] = get(ENV, key, nothing)
+            val !== nothing ? (ENV[key]=val) : Base._unsetenv(key)
+        end
+        try f()
+        finally
+            for (key,val) in old
+                val !== nothing ? (ENV[key]=val) : Base._unsetenv(key)
+            end
+        end
+    end
+    withenv(f::Function) = f() # handle empty keyvals case; see #10853
+end
+
 import Base: remotecall, remotecall_fetch, remotecall_wait, remote_do
 if VERSION < v"0.5.0-dev+431"
     for f in (:remotecall, :remotecall_fetch, :remotecall_wait, :remote_do)

test/runtests.jl

@@ -551,3 +551,9 @@ Base.remote_do(() -> true, 1) # Doesn't return anything so cannot be `@test`ed b
 
 # JuliaLang/julia#14338
 @test supertype(Int) == Signed
+
+# withenv
+@test "1234" == @compat withenv(() -> ENV["_TESTVAR"], "_TESTVAR" => 1234)
+@test "1234" == @compat withenv("_TESTVAR" => 1234) do
+    return ENV["_TESTVAR"]
+end