Move v0 to an init keyword argument for reduce, etc

The initial value `v0` has been moved to a keyword argument `init` in
`reduce`, `mapreduce`, `foldl`, `mapfoldl`, `foldr` and `mapfoldr`.

(This will allow the future addition of multiple input iterators like
we allow for `map`).

NEWS.md

@@ -710,6 +710,10 @@ Library improvements
   * Added an optimized method of `vecdot` for taking the Frobenius inner product
     of sparse matrices. ([#27470])
 
+  * The initial element `v0` in `reduce(op, v0, itr)` has been replaced with an `init`
+    optional keyword argument, as in `reduce(op, itr; init=v0)`. Similarly for `foldl`,
+    `foldr`, `mapreduce`, `mapfoldl` and `mapfoldr`. ([#27711])
+
 Compiler/Runtime improvements
 -----------------------------
 

base/Enums.jl

@@ -20,7 +20,7 @@ function membershiptest(expr, values)
     if length(values) == hi - lo + 1
         :($lo <= $expr <= $hi)
     elseif length(values) < 20
-        foldl((x1,x2)->:($x1 || ($expr == $x2)), :($expr == $(values[1])), values[2:end])
+        foldl((x1,x2)->:($x1 || ($expr == $x2)), values[2:end]; init = :($expr == $(values[1])))
     else
         :($expr in $(Set(values)))
     end

base/abstractset.jl

@@ -59,7 +59,7 @@ julia> a
 Set([7, 4, 3, 5, 1])
 ```
 """
-union!(s::AbstractSet, sets...) = foldl(union!, s, sets)
+union!(s::AbstractSet, sets...) = foldl(union!, sets; init = s)
 
 max_values(::Type) = typemax(Int)
 max_values(T::Type{<:Union{Nothing,BitIntegerSmall}}) = 1 << (8*sizeof(T))
@@ -109,7 +109,7 @@ const ∩ = intersect
 Intersect all passed in sets and overwrite `s` with the result.
 Maintain order with arrays.
 """
-intersect!(s::AbstractSet, itrs...) = foldl(intersect!, s, itrs)
+intersect!(s::AbstractSet, itrs...) = foldl(intersect!, itrs; init = s)
 intersect!(s::AbstractSet, s2::AbstractSet) = filter!(_in(s2), s)
 intersect!(s::AbstractSet, itr) =
     intersect!(s, union!(emptymutable(s, eltype(itr)), itr))
@@ -147,8 +147,8 @@ julia> a
 Set([4])
 ```
 """
-setdiff!(s::AbstractSet, itrs...) = foldl(setdiff!, s, itrs)
-setdiff!(s::AbstractSet, itr) = foldl(delete!, s, itr)
+setdiff!(s::AbstractSet, itrs...) = foldl(setdiff!, itrs; init = s)
+setdiff!(s::AbstractSet, itr) = foldl(delete!, itr; init = s)
 
 
 """
@@ -185,7 +185,7 @@ Construct the symmetric difference of the passed in sets, and overwrite `s` with
 When `s` is an array, the order is maintained.
 Note that in this case the multiplicity of elements matters.
 """
-symdiff!(s::AbstractSet, itrs...) = foldl(symdiff!, s, itrs)
+symdiff!(s::AbstractSet, itrs...) = foldl(symdiff!, itrs; init = s)
 
 function symdiff!(s::AbstractSet, itr)
     for x in itr

base/array.jl

@@ -2366,7 +2366,7 @@ _shrink_filter!(keep) = _unique_filter!(∈, pop!, keep)
 
 function _grow!(pred!, v::AbstractVector, itrs)
     filter!(pred!, v) # uniquify v
-    foldl(v, itrs) do v, itr
+    foldl(itrs; init = v) do v, itr
         mapfilter(pred!, push!, itr, v)
     end
 end

base/bitset.jl

@@ -29,7 +29,7 @@ very large integers), use [`Set`](@ref) instead.
 BitSet(itr) = union!(BitSet(), itr)
 
 # Special implementation for BitSet, which lacks a fast `length` method.
-union!(s::BitSet, itr) = foldl(push!, s, itr)
+union!(s::BitSet, itr) = foldl(push!, itr; init = s)
 
 @inline intoffset(s::BitSet) = s.offset << 6
 
@@ -274,7 +274,7 @@ intersect!(s1::BitSet, s2::BitSet) = _matched_map!(&, s1, s2)
 
 setdiff!(s1::BitSet, s2::BitSet) = _matched_map!((p, q) -> p & ~q, s1, s2)
 
-symdiff!(s::BitSet, ns) = foldl(int_symdiff!, s, ns)
+symdiff!(s::BitSet, ns) = foldl(int_symdiff!, ns; init = s)
 
 function int_symdiff!(s::BitSet, n::Integer)
     n0 = _check_bitset_bounds(n)
@@ -309,7 +309,7 @@ function last(s::BitSet)
     idx == -1 ? _throw_bitset_notempty_error() : idx + intoffset(s)
 end
 
-length(s::BitSet) = bitcount(s.bits) # = mapreduce(count_ones, +, 0, s.bits)
+length(s::BitSet) = bitcount(s.bits) # = mapreduce(count_ones, +, s.bits; init = 0)
 
 function show(io::IO, s::BitSet)
     print(io, "BitSet([")

base/compiler/ssair/passes.jl

@@ -71,7 +71,7 @@ end
 
 function compute_value_for_block(ir::IRCode, domtree::DomTree, allblocks, du, phinodes, fidx, curblock)
     curblock = find_curblock(domtree, allblocks, curblock)
-    def = reduce(max, 0, stmt for stmt in du.defs if block_for_inst(ir.cfg, stmt) == curblock)
+    def = reduce(max, stmt for stmt in du.defs if block_for_inst(ir.cfg, stmt) == curblock; init = 0)
     def == 0 ? phinodes[curblock] : val_for_def_expr(ir, def, fidx)
 end
 
@@ -680,7 +680,7 @@ function getfield_elim_pass!(ir::IRCode, domtree)
         # not to include any intermediaries that have dead uses. As a result, missing uses will only ever
         # show up in the nuses_total count.
         nleaves = length(defuse.uses) + length(defuse.defs) + length(defuse.ccall_preserve_uses)
-        nuses_total = compact.used_ssas[idx] + mapreduce(idx->compact.used_ssas[idx], +, 0, intermediaries) - length(intermediaries)
+        nuses_total = compact.used_ssas[idx] + mapreduce(idx->compact.used_ssas[idx], +, intermediaries; init = 0) - length(intermediaries)
         nleaves == nuses_total || continue
         # Find the type for this allocation
         defexpr = ir[SSAValue(idx)]

base/compiler/tfuncs.jl

@@ -616,7 +616,7 @@ function getfield_tfunc(@nospecialize(s00), @nospecialize(name))
             return rewrap_unionall(unwrapva(s.types[1]), s00)
         end
         # union together types of all fields
-        return reduce(tmerge, Bottom, map(t -> rewrap_unionall(unwrapva(t), s00), s.types))
+        return reduce(tmerge, map(t -> rewrap_unionall(unwrapva(t), s00), s.types); init = Bottom)
     end
     fld = name.val
     if isa(fld,Symbol)
@@ -688,8 +688,8 @@ function fieldtype_tfunc(@nospecialize(s0), @nospecialize(name))
         if !(Int <: name || Symbol <: name)
             return Bottom
         end
-        return reduce(tmerge, Bottom,
-                      Any[ fieldtype_tfunc(s0, Const(i)) for i = 1:length(ftypes) ])
+        return reduce(tmerge, Any[ fieldtype_tfunc(s0, Const(i)) for i = 1:length(ftypes) ];
+                      init = Bottom)
     end
 
     fld = name.val

base/compiler/typelimits.jl

@@ -20,7 +20,7 @@ function limit_tuple_type_n(@nospecialize(t), lim::Int)
     p = t.parameters
     n = length(p)
     if n > lim
-        tail = reduce(typejoin, Bottom, Any[p[lim:(n-1)]..., unwrapva(p[n])])
+        tail = reduce(typejoin, Any[p[lim:(n-1)]..., unwrapva(p[n])]; init = Bottom)
         return Tuple{p[1:(lim-1)]..., Vararg{tail}}
     end
     return t

base/compiler/typeutils.jl

@@ -85,7 +85,7 @@ _typename(a::DataType) = Const(a.name)
 function tuple_tail_elem(@nospecialize(init), ct)
     # FIXME: this is broken: it violates subtyping relations and creates invalid types with free typevars
     tmerge_maybe_vararg(@nospecialize(a), @nospecialize(b)) = tmerge(a, tvar_extent(unwrapva(b)))
-    return Vararg{widenconst(foldl(tmerge_maybe_vararg, init, ct))}
+    return Vararg{widenconst(foldl(tmerge_maybe_vararg, ct; init = init))}
 end
 
 function countunionsplit(atypes)

base/deprecated.jl

@@ -1723,6 +1723,14 @@ end
 @deprecate_moved eigs "Arpack"
 @deprecate_moved svds "Arpack"
 
+# PR #27711
+@deprecate reduce(op, v0, itr) reduce(op, itr; init=v0)
+@deprecate foldl(op, v0, itr) foldl(op, itr; init=v0)
+@deprecate foldr(op, v0, itr) foldr(op, itr; init=v0)
+@deprecate mapreduce(f, op, v0, itr) mapreduce(f, op, itr; init=v0)
+@deprecate mapfoldl(f, op, v0, itr) mapfoldl(f, op, itr; init=v0)
+@deprecate mapfoldr(f, op, v0, itr) mapfoldr(f, op, itr; init=v0)
+
 # END 0.7 deprecations
 
 # BEGIN 1.0 deprecations

base/gmp.jl

@@ -553,7 +553,7 @@ function gcdx(a::BigInt, b::BigInt)
     g, s, t
 end
 
-sum(arr::AbstractArray{BigInt}) = foldl(MPZ.add!, BigInt(0), arr)
+sum(arr::AbstractArray{BigInt}) = foldl(MPZ.add!, arr; init = BigInt(0))
 # note: a similar implementation for `prod` won't be efficient:
 # 1) the time complexity of the allocations is negligible compared to the multiplications
 # 2) assuming arr contains similarly sized BigInts, the multiplications are much more

base/intfuncs.jl

@@ -79,7 +79,7 @@ lcm(a::Integer, b::Integer) = lcm(promote(a,b)...)
 gcd(a::Integer, b::Integer...) = gcd(a, gcd(b...))
 lcm(a::Integer, b::Integer...) = lcm(a, lcm(b...))
 
-lcm(abc::AbstractArray{<:Integer}) = reduce(lcm,one(eltype(abc)),abc)
+lcm(abc::AbstractArray{<:Integer}) = reduce(lcm, abc; init = one(eltype(abc)))
 
 function gcd(abc::AbstractArray{<:Integer})
     a = zero(eltype(abc))