Sorta working new lengths.

original commit: dcb8fa2b5e74d0da9e6a8237cf5c5862681e00c8

pkgs/typed-racket-pkgs/typed-racket-lib/typed-racket/static-contracts/combinators/derived.rkt

@@ -14,18 +14,11 @@
 (define promise?/sc (flat/sc #'promise?))
 
 (define list?/sc (flat/sc #'list?))
-(define empty-list/sc (flat/sc #'null?))
-(define (list-length/sc n)
-  (if (equal? 0 n)
-      empty-list/sc
-      (and/sc list?/sc (flat/sc #`(λ (l) (= #,n (length l)))))))
 
 (define set?/sc (flat/sc #'set?))
 (define empty-set/sc (and/sc set?/sc (flat/sc #'set-empty?)))
 
 (define vector?/sc (flat/sc #'vector?))
-(define (vector-length/sc n) (and/sc vector?/sc (flat/sc #`(λ (v) (= #,n (vector-length v))))))
-(define empty-vector/sc (vector-length/sc 0))
 
 (define hash?/sc (flat/sc #'hash?))
 (define empty-hash/sc (and/sc hash?/sc (flat/sc #'(λ (h) (zero? (hash-count h))))))

pkgs/typed-racket-pkgs/typed-racket-lib/typed-racket/static-contracts/combinators/lengths.rkt

@@ -0,0 +1,50 @@
+#lang racket/base
+
+;; Static contracts for list and vector lengths.
+;; These are used during optimizations as simplifications.
+;; Ex: (list/sc any/sc) => (list-length/sc 1)
+
+(require
+  "../structures.rkt"
+  "simple.rkt"
+  racket/match
+  (for-template racket/base))
+
+(provide
+  list-length/sc
+  vector-length/sc
+  empty-list/sc
+  empty-vector/sc)
+
+
+(define (length-contract-write-proc v port mode)
+  (match-define (length-contract name length stx) v)
+  (define-values (open close)
+    (if (equal? mode 0)
+        (values "(" ")")
+        (values "#<" ">")))
+  (display open port)
+  (fprintf port "~a/sc" name)
+  (display " " port)
+  (write length port)
+  (display close port))
+
+
+(struct length-contract static-contract (name length syntax)
+        #:methods gen:sc
+         [(define (sc-map v f) v)
+          (define (sc->contract v f) (length-contract-syntax v))
+          (define (sc->constraints v f) 'flat)]
+        #:methods gen:custom-write [(define write-proc length-contract-write-proc)])
+
+(define (list-length/sc n)
+  (if (equal? 0 n)
+      empty-list/sc
+      (length-contract 'length n
+                       #`(λ (l) (and (list? l) (= #,n (length l)))))))
+(define empty-list/sc (flat/sc #'null?))
+
+(define (vector-length/sc n)
+  (length-contract 'vector-length n
+                   #`(λ (l) (and (vector? l) (= #,n (vector-length l))))))
+(define empty-vector/sc (vector-length/sc 0))