Merge pull request #22659 from JuliaLang/jb/loopvars

RFC: deprecate `for` loop vars that overwrite outer vars (#22314)

NEWS.md

@@ -70,6 +70,11 @@ Language changes
     warning, so that this syntax can be disallowed or given a new meaning in a
     future version ([#5148]).
 
+  * In `for i = ...`, if a local variable `i` already existed it would be overwritten
+    during the loop. This behavior is deprecated, and in the future `for` loop variables
+    will always be new variables local to the loop ([#22314]).
+    The old behavior of overwriting an existing variable is available via `for outer i = ...`.
+
   * In `for i in x`, `x` used to be evaluated in a new scope enclosing the `for` loop.
     Now it is evaluated in the scope outside the `for` loop.
 

base/deprecated.jl

@@ -88,7 +88,7 @@ function firstcaller(bt::Array{Ptr{Void},1}, funcsyms)
     lkup = StackTraces.UNKNOWN
     for frame in bt
         lkups = StackTraces.lookup(frame)
-        for lkup in lkups
+        for outer lkup in lkups
             if lkup == StackTraces.UNKNOWN
                 continue
             end
@@ -1067,7 +1067,7 @@ function partial_linear_indexing_warning_lookup(nidxs_remaining)
         caller = StackTraces.UNKNOWN
         for frame in bt
             lkups = StackTraces.lookup(frame)
-            for caller in lkups
+            for outer caller in lkups
                 if caller == StackTraces.UNKNOWN
                     continue
                 end

base/docs/utils.jl

@@ -289,7 +289,7 @@ function levsort(search, candidates)
     scores = map(cand -> (levenshtein(search, cand), -fuzzyscore(search, cand)), candidates)
     candidates = candidates[sortperm(scores)]
     i = 0
-    for i = 1:length(candidates)
+    for outer i = 1:length(candidates)
         levenshtein(search, candidates[i]) > 3 && break
     end
     return candidates[1:i]
@@ -328,7 +328,7 @@ printmatches(args...; cols = displaysize(STDOUT)[2]) = printmatches(STDOUT, args
 function print_joined_cols(io::IO, ss, delim = "", last = delim; cols = displaysize(io)[2])
     i = 0
     total = 0
-    for i = 1:length(ss)
+    for outer i = 1:length(ss)
         total += length(ss[i])
         total + max(i-2,0)*length(delim) + (i>1 ? 1 : 0)*length(last) > cols && (i-=1; break)
     end

base/inference.jl

@@ -3618,9 +3618,9 @@ end
 function type_annotate!(sv::InferenceState)
     # remove all unused ssa values
     gt = sv.src.ssavaluetypes
-    for i = 1:length(gt)
-        if gt[i] === NF
-            gt[i] = Union{}
+    for j = 1:length(gt)
+        if gt[j] === NF
+            gt[j] = Union{}
         end
     end
 
@@ -3671,9 +3671,9 @@ function type_annotate!(sv::InferenceState)
     end
 
     # finish marking used-undef variables
-    for i = 1:nslots
-        if undefs[i]
-            src.slotflags[i] |= Slot_UsedUndef
+    for j = 1:nslots
+        if undefs[j]
+            src.slotflags[j] |= Slot_UsedUndef
         end
     end
     nothing
@@ -4650,10 +4650,10 @@ function inlineable(@nospecialize(f), @nospecialize(ft), e::Expr, atypes::Vector
     if !isempty(stmts) && !propagate_inbounds
         # avoid redundant inbounds annotations
         s_1, s_end = stmts[1], stmts[end]
-        i = 2
-        while length(stmts) > i && ((isa(s_1,Expr)&&s_1.head===:line) || isa(s_1,LineNumberNode))
-            s_1 = stmts[i]
-            i += 1
+        si = 2
+        while length(stmts) > si && ((isa(s_1,Expr)&&s_1.head===:line) || isa(s_1,LineNumberNode))
+            s_1 = stmts[si]
+            si += 1
         end
         if isa(s_1, Expr) && s_1.head === :inbounds && s_1.args[1] === false &&
             isa(s_end, Expr) && s_end.head === :inbounds && s_end.args[1] === :pop

base/intfuncs.jl

@@ -833,9 +833,8 @@ end
 
 function factorial(n::Integer)
     n < 0 && throw(DomainError(n, "`n` must be nonnegative."))
-    local f::typeof(n*n), i::typeof(n*n)
-    f = 1
-    for i = 2:n
+    f::typeof(n*n) = 1
+    for i::typeof(n*n) = 2:n
         f *= i
     end
     return f

base/linalg/triangular.jl

@@ -1836,7 +1836,8 @@ function logm(A0::UpperTriangular{T}) where T<:Union{Float64,Complex{Float64}}
         d4 = norm(AmI^4, 1)^(1/4)
         alpha3 = max(d3, d4)
         if alpha3 <= theta[tmax]
-            for j = 3:tmax
+            local j
+            for outer j = 3:tmax
                 if alpha3 <= theta[j]
                     break
                 end
@@ -1856,7 +1857,7 @@ function logm(A0::UpperTriangular{T}) where T<:Union{Float64,Complex{Float64}}
             eta = min(alpha3, alpha4)
             if eta <= theta[tmax]
                 j = 0
-                for j = 6:tmax
+                for outer j = 6:tmax
                     if eta <= theta[j]
                         m = j
                         break
@@ -2035,7 +2036,8 @@ function invsquaring(A0::UpperTriangular, theta)
         d4 = norm(AmI^4, 1)^(1/4)
         alpha3 = max(d3, d4)
         if alpha3 <= theta[tmax]
-            for j = 3:tmax
+            local j
+            for outer j = 3:tmax
                 if alpha3 <= theta[j]
                     break
                 elseif alpha3 / 2 <= theta[5] && p < 2
@@ -2059,7 +2061,7 @@ function invsquaring(A0::UpperTriangular, theta)
             eta = min(alpha3, alpha4)
             if eta <= theta[tmax]
                 j = 0
-                for j = 6:tmax
+                for outer j = 6:tmax
                     if eta <= theta[j]
                         m = j
                         break

base/markdown/Markdown.jl

@@ -56,8 +56,8 @@ macro doc_str(s::AbstractString, t...)
     docexpr(__source__, __module__, s, t...)
 end
 
-function Base.display(d::Base.REPL.REPLDisplay, md::Vector{MD})
-    for md in md
+function Base.display(d::Base.REPL.REPLDisplay, mds::Vector{MD})
+    for md in mds
         display(d, md)
     end
 end

base/markdown/render/latex.jl

@@ -46,8 +46,8 @@ function latexinline(io::IO, code::Code)
 end
 
 function latex(io::IO, md::Paragraph)
-    for md in md.content
-        latexinline(io, md)
+    for mdc in md.content
+        latexinline(io, mdc)
     end
     println(io)
     println(io)

base/markdown/render/rst.jl

@@ -90,8 +90,8 @@ end
 
 function rst(io::IO, l::LaTeX)
     println(io, ".. math::\n")
-    for l in lines(l.formula)
-        println(io, "    ", l)
+    for line in lines(l.formula)
+        println(io, "    ", line)
     end
 end
 

base/multidimensional.jl

@@ -841,9 +841,9 @@ end
 
 @noinline function _accumulate!(op, B, A, R1, ind, R2)
     # Copy the initial element in each 1d vector along dimension `axis`
-    i = first(ind)
+    ii = first(ind)
     @inbounds for J in R2, I in R1
-        B[I, i, J] = A[I, i, J]
+        B[I, ii, J] = A[I, ii, J]
     end
     # Accumulate
     @inbounds for J in R2, i in first(ind)+1:last(ind), I in R1

base/printf.jl

@@ -57,7 +57,7 @@ function parse(s::AbstractString)
     i = 1
     while i < length(list)
         if isa(list[i],AbstractString)
-            for j = i+1:length(list)
+            for outer j = i+1:length(list)
                 if !isa(list[j],AbstractString)
                     j -= 1
                     break

base/sparse/linalg.jl

@@ -301,8 +301,8 @@ function A_rdiv_B!(A::SparseMatrixCSC{T}, D::Diagonal{T}) where T
         if iszero(ddj)
             throw(LinAlg.SingularException(j))
         end
-        for k in nzrange(A, j)
-            nonz[k] /= ddj
+        for i in nzrange(A, j)
+            nonz[i] /= ddj
         end
     end
     A

doc/src/manual/variables-and-scoping.md

@@ -435,7 +435,7 @@ julia> Fs[2]()
 2
 ```
 
-`for` loops will reuse existing variables for its iteration variable:
+A `for` loop or comprehension iteration variable is always a new variable:
 
 ```jldoctest
 julia> i = 0;
@@ -444,18 +444,20 @@ julia> for i = 1:3
        end
 
 julia> i
-3
+0
 ```
 
-However, comprehensions do not do this, and always freshly allocate their iteration variables:
+However, it is occasionally useful to reuse an existing variable as the iteration variable.
+This can be done conveniently by adding the keyword `outer`:
 
 ```jldoctest
-julia> x = 0;
+julia> i = 0;
 
-julia> [ x for x = 1:3 ];
+julia> for outer i = 1:3
+       end
 
-julia> x
-0
+julia> i
+3
 ```
 
 ## Constants

src/julia-parser.scm

@@ -1611,7 +1611,18 @@
 
 ;; as above, but allows both "i=r" and "i in r"
 (define (parse-iteration-spec s)
-  (let* ((lhs (parse-pipes s))
+  (let* ((outer? (if (eq? (peek-token s) 'outer)
+                     (begin
+                       (take-token s)
+                       (let ((nxt (peek-token s)))
+                         (if (or (memq nxt '(= in ∈))
+                                 (not (symbol? nxt))
+                                 (operator? nxt))
+                             (begin (ts:put-back! s 'outer #t)
+                                    #f)
+                             #t)))
+                     #f))
+         (lhs (parse-pipes s))
          (t   (peek-token s)))
     (cond ((memq t '(= in ∈))
            (take-token s)
@@ -1621,7 +1632,9 @@
                  ;; should be: (error "invalid iteration specification")
                  (syntax-deprecation s (string "for " (deparse `(= ,lhs ,rhs)) " " t)
                                      (string "for " (deparse `(= ,lhs ,rhs)) "; " t)))
-             `(= ,lhs ,rhs)))
+             (if outer?
+                 `(= (outer ,lhs) ,rhs)
+                 `(= ,lhs ,rhs))))
           ((and (eq? lhs ':) (closing-token? t))
            ':)
           (else (error "invalid iteration specification")))))