Allow omitting type annotation in most of the for: forms.

collects/tests/typed-scheme/succeed/for-no-body-anns.rkt

@@ -0,0 +1,9 @@
+#lang typed/racket
+
+(ann (for/list ([#{i : Number} (in-list '(1 2 3))]) i) (Listof Number))
+(ann (for/list: ([i : Number (in-list '(1 2 3))]) i) (Listof Number))
+
+(+ (for/fold: ([acc : Number 0])
+       ([i (in-list '(1 2 3))])
+     (+ i acc))
+   1)

collects/typed-scheme/base-env/annotate-classes.rkt

@@ -133,3 +133,11 @@
                        (~or rest:annotated-star-rest rest:annotated-dots-rest)))
            #:with ann-formals #'(n.ann-name ... . rest.ann-name)
            #:with (arg-ty ...) #'(n.ty ... . rest.formal-ty)))
+
+(define-splicing-syntax-class standalone-annotation
+  #:literals (:)
+  (pattern (~seq : t)
+           #:with ty #'t))
+(define-splicing-syntax-class optional-standalone-annotation
+  (pattern (~optional a:standalone-annotation)
+           #:with ty (if (attribute a) #'a.ty #f)))

collects/typed-scheme/base-env/prims.rkt

@@ -491,6 +491,11 @@ This file defines two sorts of primitives. All of them are provided into any mod
               (when guard
                 #,(loop #'(rest ...))))])))]))
 
+(define-for-syntax (maybe-annotate-body body ty)
+  (if (syntax-e ty)
+      (syntax-property body 'type-ascription ty)
+      body))
+
 ;; Handling #:when clauses manually, like we do with for: above breaks
 ;; the semantics of for/list and co.
 ;; We must leave it to the untyped versions of the macros.
@@ -500,19 +505,17 @@ This file defines two sorts of primitives. All of them are provided into any mod
 (define-for-syntax (define-for-variant name)
   (lambda (stx)
     (syntax-parse stx #:literals (:)
-      [(_ : ty
+      [(_ a:optional-standalone-annotation
           (clause:for-clause ...)
           c:expr ...)
-       (syntax-property
+       (maybe-annotate-body
         (quasisyntax/loc stx
           (#,name
            (clause.expand ... ...)
-          #,@(syntax-property
+           #,@(maybe-annotate-body
                #'(c ...)
-              'type-ascription
-              #'ty)))
-        'type-ascription
-        #'ty)])))
+               #'a.ty)))
+        #'a.ty)])))
 (define-syntax (define-for-variants stx)
   (syntax-parse stx
     [(_ (name untyped-name) ...)
@@ -537,17 +540,16 @@ This file defines two sorts of primitives. All of them are provided into any mod
 ;; clauses with these 2.
 (define-syntax (for/lists: stx)
   (syntax-parse stx #:literals (:)
-    [(_ : ty
+    [(_ a:optional-standalone-annotation
         ((var:optionally-annotated-name) ...)
         (clause:for-clause ...)
         c:expr ...)
-     (syntax-property
+     (maybe-annotate-body
       (quasisyntax/loc stx
         (for/lists (var.ann-name ...)
           (clause.expand ... ...)
           c ...))
-      'type-ascription
-      #'ty)]))
+      #'a.ty)]))
 (define-syntax (for/fold: stx)
   (syntax-parse stx #:literals (:)
     [(_ : ty
@@ -591,15 +593,14 @@ This file defines two sorts of primitives. All of them are provided into any mod
 (define-for-syntax (define-for*-variant name)
   (lambda (stx)
     (syntax-parse stx #:literals (:)
-      [(_ : ty
+      [(_ a:optional-standalone-annotation
           (clause:for-clause ...)
           c:expr ...)
-       (syntax-property
+       (maybe-annotate-body
         (quasisyntax/loc stx
           (#,name (clause.expand ... ...)
                   c ...))
-        'type-ascription
-        #'ty)])))
+        #'a.ty)])))
 (define-syntax (define-for*-variants stx)
   (syntax-parse stx
     [(_ (name no-*-name) ...)