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commit 579815512eb2b49d4a1311bf5e4d3a1126624d5e
parent ce1bc6b90ffce51ee5fabb7e0c1255b0bb63c249
Author: Stephen Chang <stchang@ccs.neu.edu>
Date:   Wed, 20 Apr 2016 19:18:33 -0400

for/fold produces better err msg

Diffstat:
M
tapl/mlish.rkt
 | 
6
+++++-
M
tapl/tests/mlish-tests.rkt
 | 
3
+++

2 files changed, 8 insertions(+), 1 deletion(-)
diff --git a/tapl/mlish.rkt b/tapl/mlish.rkt
@@ -924,7 +924,11 @@
    #:with ([e- (ty)] ...) (⇑s (e ...) as Sequence)
    #:with [(acc- x- ...) body- ty_body] 
           (infer/ctx+erase #'([acc : ty_init][x : ty] ...) #'body)
-   #:when (typecheck? #'ty_body #'ty_init)
+   #:fail-unless (typecheck? #'ty_body #'ty_init)
+                 (type-error #:src stx
+                  #:msg 
+                  "for/fold: Type of body and initial accumulator must be the same, given ~a and ~a"
+                  #'ty_init #'ty_body)
    (⊢ (for/fold ([acc- init-]) ([x- e-] ...) body-) : ty_body)])
 
 (define-typed-syntax for/hash
diff --git a/tapl/tests/mlish-tests.rkt b/tapl/tests/mlish-tests.rkt
@@ -390,6 +390,9 @@
 (check-type RT2 : (→/test {X Y} Y X (List X) (RecoTest X Y)))
 (check-type RT3 : (→/test X Y (RecoTest X Y)))
 
+(typecheck-fail (for/fold ([x 1]) () "hello") 
+ #:with-msg "for/fold: Type of body and initial accumulator must be the same, given Int and String")
+
 ; ext-stlc tests --------------------------------------------------
 
 ; tests for stlc extensions