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fresh variables on each iteration in nested loop and generator syntax
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fixes #330
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@JeffBezanson
JeffBezanson committed Mar 15, 2018
1 parent 71fbc70 commit 39ab43a
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4 changes: 4 additions & 0 deletions NEWS.md
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Expand Up @@ -151,6 +151,10 @@ Language changes
* 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.

* In `for i in x, j in y`, all variables now have fresh bindings on each iteration of the
innermost loop. For example, an assignment to `i` will not be visible on the next `j`
loop iteration ([#330]).

* Variable bindings local to `while` loop bodies are now freshly allocated on each loop iteration,
matching the behavior of `for` loops.

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20 changes: 19 additions & 1 deletion doc/src/manual/control-flow.md
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Expand Up @@ -527,7 +527,25 @@ julia> for i = 1:2, j = 3:4
(2, 4)
```

A `break` statement inside such a loop exits the entire nest of loops, not just the inner one.
With this syntax, iterables may still refer to outer loop variables; e.g. `for i = 1:n, j = 1:i`
is valid.
However a `break` statement inside such a loop exits the entire nest of loops, not just the inner one.
Both variables (`i` and `j`) are set to their current iteration values each time the inner loop runs.
Therefore, assignments to `i` will not be visible to subsequent iterations:

```jldoctest
julia> for i = 1:2, j = 3:4
println((i, j))
i = 0
end
(1, 3)
(1, 4)
(2, 3)
(2, 4)
```

If this example were rewritten to use a `for` keyword for each variable, then the output would
be different: the second and fourth values would contain `0`.

## Exception Handling

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144 changes: 83 additions & 61 deletions src/julia-syntax.scm
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Expand Up @@ -1577,50 +1577,100 @@
;; If false, this will try to warn only for uses of the last value after the loop.
(define *warn-all-loop-vars* #f)

(define (expand-for while lhs X body)
;; (for (= lhs X) body)
(let* ((coll (make-ssavalue))
(state (gensy))
(outer? (and (pair? lhs) (eq? (car lhs) 'outer)))
(lhs (if outer? (cadr lhs) lhs)))
`(block (= ,coll ,(expand-forms X))
(= ,state (call (top start) ,coll))
;; TODO avoid `local declared twice` error from this
;;,@(if outer? `((local ,lhs)) '())
,@(if outer? `((require-existing-local ,lhs)) '())
,(expand-forms
`(,while
(call (top not_int) (call (core typeassert) (call (top done) ,coll ,state) (core Bool)))
(block
;; NOTE: enable this to force loop-local var
#;,@(map (lambda (v) `(local ,v)) (lhs-vars lhs))
,@(if (not outer?)
(map (lambda (v) `(warn-if-existing ,v)) (lhs-vars lhs))
'())
,(lower-tuple-assignment (list lhs state)
`(call (top next) ,coll ,state))
,body))))))
(define (expand-for lhss itrs body)
(define (outer? x) (and (pair? x) (eq? (car x) 'outer)))
(let ((copied-vars ;; variables not declared `outer` are copied in the innermost loop
;; TODO: maybe filter these to remove vars not assigned in the loop
(delete-duplicates
(apply append
(map lhs-vars
(filter (lambda (x) (not (outer? x))) (butlast lhss)))))))
`(break-block
loop-exit
,(let nest ((lhss lhss)
(itrs itrs))
(if (null? lhss)
body
(let* ((coll (make-ssavalue))
(state (gensy))
(outer (outer? (car lhss)))
(lhs (if outer (cadar lhss) (car lhss)))
(body
`(block
;; NOTE: enable this to force loop-local var
#;,@(map (lambda (v) `(local ,v)) (lhs-vars lhs))
,@(if (not outer)
(map (lambda (v) `(warn-if-existing ,v)) (lhs-vars lhs))
'())
,(lower-tuple-assignment (list lhs state)
`(call (top next) ,coll ,state))
,(nest (cdr lhss) (cdr itrs))))
(body
(if (null? (cdr lhss))
`(break-block
loop-cont
(let (block ,@(map (lambda (v) `(= ,v ,v)) copied-vars))
,body))
`(scope-block ,body))))
`(block (= ,coll ,(car itrs))
(= ,state (call (top start) ,coll))
;; TODO avoid `local declared twice` error from this
;;,@(if outer `((local ,lhs)) '())
,@(if outer `((require-existing-local ,lhs)) '())
(_while
(call (top not_int) (call (core typeassert) (call (top done) ,coll ,state) (core Bool)))
,body))))))))

;; wrap `expr` in a function appropriate for consuming values from given ranges
(define (func-for-generator-ranges expr range-exprs)
(define (func-for-generator-ranges expr range-exprs flat outervars)
(let* ((vars (map cadr range-exprs))
(argname (if (and (length= vars 1) (symbol? (car vars)))
(car vars)
(gensy)))
(myvars (lhs-vars `(tuple ,@vars)))
(splat (cond ((eq? argname (car vars)) '())
((length= vars 1)
`(,@(map (lambda (v) `(local ,v)) (lhs-vars (car vars)))
`(,@(map (lambda (v) `(local ,v)) myvars)
(= ,(car vars) ,argname)))
(else
`(,@(map (lambda (v) `(local ,v)) (lhs-vars `(tuple ,@vars)))
`(,@(map (lambda (v) `(local ,v)) myvars)
(= (tuple ,@vars) ,argname))))))
(if (and (null? splat)
(length= expr 3) (eq? (car expr) 'call)
(eq? (caddr expr) argname)
(not (dotop? (cadr expr)))
(not (expr-contains-eq argname (cadr expr))))
(cadr expr) ;; eta reduce `x->f(x)` => `f`
`(-> ,argname (block ,@splat ,expr)))))
(let ((expr (cond ((and flat (pair? expr) (eq? (car expr) 'generator))
(expand-generator expr #f (delete-duplicates (append outervars myvars))))
((and flat (pair? expr) (eq? (car expr) 'flatten))
(expand-generator (cadr expr) #t (delete-duplicates (append outervars myvars))))
((pair? outervars)
`(let (block ,@(map (lambda (v) `(= ,v ,v)) outervars))
,expr))
(else expr))))
`(-> ,argname (block ,@splat ,expr))))))

(define (expand-generator e flat outervars)
(let* ((expr (cadr e))
(filt? (eq? (car (caddr e)) 'filter))
(range-exprs (if filt? (cddr (caddr e)) (cddr e)))
(ranges (map caddr range-exprs))
(iter (if (length= ranges 1)
(car ranges)
`(call (top product) ,@ranges)))
(iter (if filt?
`(call (|.| (top Iterators) 'Filter)
,(func-for-generator-ranges (cadr (caddr e)) range-exprs #f '())
,iter)
iter))
(gen `(call (top Generator)
,(func-for-generator-ranges expr range-exprs flat outervars)
,iter)))
(expand-forms
(if flat
`(call (top Flatten) ,gen)
gen))))

(define (ref-to-view expr)
(if (and (pair? expr) (eq? (car expr) 'ref))
Expand Down Expand Up @@ -2217,12 +2267,6 @@
(break-block loop-cont
(scope-block ,(blockify (expand-forms (caddr e))))))))

'inner-while
(lambda (e)
`(_while ,(expand-forms (cadr e))
(break-block loop-cont
(scope-block ,(blockify (expand-forms (caddr e)))))))

'break
(lambda (e)
(if (pair? (cdr e))
Expand All @@ -2233,16 +2277,10 @@

'for
(lambda (e)
(let nest ((ranges (if (eq? (car (cadr e)) 'block)
(cdr (cadr e))
(list (cadr e))))
(first #t))
(expand-for (if first 'while 'inner-while)
(cadr (car ranges))
(caddr (car ranges))
(if (null? (cdr ranges))
(caddr e) ;; body
(nest (cdr ranges) #f)))))
(let ((ranges (if (eq? (car (cadr e)) 'block)
(cdr (cadr e))
(list (cadr e)))))
(expand-forms (expand-for (map cadr ranges) (map caddr ranges) (caddr e)))))

'&& (lambda (e) (expand-forms (expand-and e)))
'|\|\|| (lambda (e) (expand-forms (expand-or e)))
Expand Down Expand Up @@ -2346,26 +2384,10 @@
(return (expand-forms `(call transpose ,(cadr e))))))

'generator
(lambda (e)
(let* ((expr (cadr e))
(filt? (eq? (car (caddr e)) 'filter))
(range-exprs (if filt? (cddr (caddr e)) (cddr e)))
(ranges (map caddr range-exprs))
(iter (if (length= ranges 1)
(car ranges)
`(call (top product) ,@ranges)))
(iter (if filt?
`(call (|.| (top Iterators) 'Filter)
,(func-for-generator-ranges (cadr (caddr e)) range-exprs)
,iter)
iter)))
(expand-forms
`(call (top Generator)
,(func-for-generator-ranges expr range-exprs)
,iter))))
(lambda (e) (expand-generator e #f '()))

'flatten
(lambda (e) (expand-forms `(call (top Flatten) ,(cadr e))))
(lambda (e) (expand-generator (cadr e) #t '()))

'comprehension
(lambda (e)
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24 changes: 24 additions & 0 deletions test/functional.jl
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Expand Up @@ -144,6 +144,30 @@ end

@test [(i,j) for i=1:3 for j=1:i if j>1] == [(2,2), (3,2), (3,3)]

# issue #330
@test [(t=(i,j); i=nothing; t) for i = 1:3 for j = 1:i] ==
[(1, 1), (2, 1), (2, 2), (3, 1), (3, 2), (3, 3)]

@test map(collect, (((t=(i,j); i=nothing; t) for j = 1:i) for i = 1:3)) ==
[[(1, 1)],
[(2, 1), (nothing, 2)],
[(3, 1), (nothing, 2), (nothing, 3)]]

let a = []
for x = 1:3, y = 1:3
push!(a, x)
x = 0
end
@test a == [1,1,1,2,2,2,3,3,3]
end

let i, j
for outer i = 1:2, j = 1:0
end
@test i == 2
@test !@isdefined(j)
end

# issue #18707
@test [(q,d,n,p) for q = 0:25:100
for d = 0:10:100-q
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