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fix for/fold/derived examples

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The previous for/fold/derived examples in the docs
incorrectly expanded, placing the entire body of the
user defined for loop into a let expression inside of
for/fold/derived. This meant that break clauses (i.e. #:break
or #:final) that appeared in the body of the user-defined
for loop were not visible to the underlying for/fold/derived
macro after expansion and therefore usages of #:break or #:final
incorrectly resulted in syntax errors (since with the incorrect
expansion, they were seemingly misplaced keywords inside of a let).

With this PR the for/fold/derived examples in the docs now
expand correctly into a form that mirrors the actual
expected syntax of for loops:

(user-defined-for (for-clause ...) body-or-break ... body)

==(now expands more or less into)==>

(for/fold/derived (for-clause ...) body-or-break ... body)

Or in other words, the body of the user defined for loop now correctly
expands directly into the body of for/fold/derived.
This commit is contained in:
Andrew Kent 2017-01-24 12:57:34 -05:00 committed by Matthew Flatt
parent ec5923a5c7
commit 98b167d3a0
1 changed files with 23 additions and 20 deletions
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43
pkgs/racket-doc/scribblings/reference/for.scrbl
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@ -379,13 +379,15 @@ source for all syntax errors.
@mz-examples[#:eval for-eval
(define-syntax (for/digits stx)
(syntax-case stx ()
[(_ clauses . defs+exprs)
[(_ clauses body ... tail-expr)
(with-syntax ([original stx])
#'(let-values
([(n k)
(for/fold/derived original ([n 0] [k 1]) clauses
(define d (let () . defs+exprs))
(values (+ n (* d k)) (* k 10)))])
([(n k)
(for/fold/derived
original ([n 0] [k 1])
clauses
body ...
(values (+ n (* tail-expr k)) (* k 10)))])
n))]))
@code:comment{If we misuse for/digits, we can get good error reporting}
@ -405,16 +407,16 @@ source for all syntax errors.
@code:comment{Another example: compute the max during iteration:}
(define-syntax (for/max stx)
(syntax-case stx ()
[(_ clauses . defs+exprs)
(with-syntax ([original stx])
#'(for/fold/derived original
([current-max -inf.0])
clauses
(define maybe-new-max
(let () . defs+exprs))
(if (> maybe-new-max current-max)
maybe-new-max
current-max)))]))
[(_ clauses body ... tail-expr)
(with-syntax ([original stx])
#'(for/fold/derived original
([current-max -inf.0])
clauses
body ...
(define maybe-new-max tail-expr)
(if (> maybe-new-max current-max)
maybe-new-max
current-max)))]))
(for/max ([n '(3.14159 2.71828 1.61803)]
[s '(-1 1 1)])
(* n s))
@ -429,13 +431,14 @@ Like @racket[for*/fold], but the extra @racket[orig-datum] is used as the source
@mz-examples[#:eval for-eval
(define-syntax (for*/digits stx)
(syntax-case stx ()
[(_ clauses . defs+exprs)
[(_ clauses body ... tail-expr)
(with-syntax ([original stx])
#'(let-values
([(n k)
(for*/fold/derived original ([n 0] [k 1]) clauses
(define d (let () . defs+exprs))
(values (+ n (* d k)) (* k 10)))])
([(n k)
(for*/fold/derived original ([n 0] [k 1])
clauses
body ...
(values (+ n (* tail-expr k)) (* k 10)))])
n))]))
(eval:error