Adjust the expansion of def to permit recursive top-level definitions

The order of definitions is much more relevant at the top level, since
it evaluates forms one by one, so expand to the definition of a typed
identifier before the definition of the value itself.

Reported by @iitalics in #74.

Author: lexi-lambda

hackett-lib/hackett/private/base.rkt

@@ -392,10 +392,10 @@
    #:with id- (generate-temporary #'id)
    #:with t_reduced (if (attribute exact?) #'t.expansion (type-reduce-context #'t.expansion))
    #`(begin-
-       (define- id- (:/use #,((attribute t.scoped-binding-introducer) #'e) t_reduced #:exact))
        #,(indirect-infix-definition
           #'(define-syntax- id (make-typed-var-transformer #'id- (quote-syntax t_reduced)))
-          (attribute fixity.fixity)))]
+          (attribute fixity.fixity))
+       (define- id- (:/use #,((attribute t.scoped-binding-introducer) #'e) t_reduced #:exact)))]
   [(_ id:id
       {~optional fixity:fixity-annotation}
       e:expr)
@@ -404,11 +404,10 @@
    #:do [(match-define-values [(list id-) e-] (τ⇐/λ! #'e #'t_e (list (cons #'id #'t_e))))]
    #:with t_gen (type-reduce-context (generalize (apply-current-subst #'t_e)))
    #`(begin-
-       (define- #,id- #,e-)
        #,(indirect-infix-definition
-          #`(define-syntax- id
-              (make-typed-var-transformer (quote-syntax #,id-) (quote-syntax t_gen)))
-          (attribute fixity.fixity)))])
+          #`(define-syntax- id (make-typed-var-transformer (quote-syntax #,id-) (quote-syntax t_gen)))
+          (attribute fixity.fixity))
+       (define- #,id- #,e-))])
 
 (begin-for-syntax
   (struct todo-item (full summary) #:prefab))

hackett-test/info.rkt

@@ -5,6 +5,8 @@
 (define deps
   '("base"
     "hackett-lib"
+    "rackunit-lib"
+    "sandbox-lib"
     "testing-util-lib"))
 (define build-deps
   '())

hackett-test/tests/hackett/integration/top-level-recursive-def.rkt

@@ -0,0 +1,17 @@
+#lang racket/base
+
+; Ensure recursive definitions work at the top level.
+
+(require racket/sandbox
+         rackunit
+         syntax/strip-context)
+
+(define hackett-evaluator (make-evaluator 'hackett))
+(define (hackett-eval stx) (hackett-evaluator (strip-context stx)))
+
+(hackett-eval #'(require hackett))
+(hackett-eval #'(defn fac : {Integer -> Integer}
+                  [[0] 1]
+                  [[x] {x * (fac {x - 1})}]))
+
+(check-equal? (hackett-eval #'(fac 6)) (hackett-eval #'720))