From 2af8d9276be43c4541183245cbba00c96e4972de Mon Sep 17 00:00:00 2001
From: Sergiu Ivanov <sivanov@colimite.fr>
Date: Sun, 13 Feb 2022 19:33:04 +0100
Subject: [PATCH] utils: Typed lists-transpose.

---
 scribblings/utils.scrbl | 14 ++++++++++++++
 utils.rkt               | 36 ++++++++++++------------------------
 2 files changed, 26 insertions(+), 24 deletions(-)

diff --git a/scribblings/utils.scrbl b/scribblings/utils.scrbl
index 5c3555a..22d298c 100644
--- a/scribblings/utils.scrbl
+++ b/scribblings/utils.scrbl
@@ -386,6 +386,20 @@ consisting of the second halves.
 (multi-split-at '((1 2 3) (a b c)) 2)
 ]}
 
+@defproc[(lists-transpose [lists (List (Listof a) ... a)])
+			  (Listof (List a ... a))]{
+
+Transposes a list of lists.  The length of the resulting list is the length of
+the shortest list in @racket[lists].
+
+This function is essentially @racket[in-parallel], wrapped in
+a couple conversions.
+
+@examples[#:eval utils-evaluator
+(lists-transpose '((a b) (1 2)))
+(lists-transpose '((a b) (1 2 3) (#t)))
+]}
+
 @section{Functions and procedures}
 
 @section{Randomness}
diff --git a/utils.rkt b/utils.rkt
index 1a313ed..45ae6fa 100644
--- a/utils.rkt
+++ b/utils.rkt
@@ -24,7 +24,7 @@
    list-sets->list-strings pretty-print-set pretty-print-set-sets
    update-vertices/unweighted update-graph dotit collect-by-key
    collect-by-key/sets ht-values/list->set hash->list/ordered
-   multi-split-at)
+   multi-split-at lists-transpose)
 
   (define-type Variable Symbol)
   (define-type (VariableMapping A) (Immutable-HashTable Variable A))
@@ -445,6 +445,15 @@
     (test-case "multi-split-at"
       (define-values (l1 l2) (multi-split-at '((1 2 3) (a b c)) 2))
       (check-equal? l1 '((1 2) (a b))) (check-equal? l2 '((3) (c)))))
+
+  ;; https://racket.discourse.group/t/get-to-type-apply-in-parallel-lst/683
+  (: lists-transpose (All (a ...) (-> (List (Listof a) ... a) (Listof (List a ... a)))))
+  (define (lists-transpose lists)
+    (sequence->list (in-values-sequence (apply in-parallel lists))))
+
+  (module+ test
+  (test-case "lists-transpose"
+    (check-equal? (lists-transpose '((1 2) (a b))) '((1 a) (2 b)))))
   )
 
 (require 'typed)
@@ -455,14 +464,13 @@
          list-sets->list-strings pretty-print-set pretty-print-set-sets
          update-vertices/unweighted update-graph dotit collect-by-key
          collect-by-key/sets ht-values/list->set hash->list/ordered
-         multi-split-at)
+         multi-split-at lists-transpose)
 
 ;;; Untyped section.
 
 (provide
  ;; Functions
- (contract-out [lists-transpose (-> (listof (listof any/c)) (listof (listof any/c)))]
-               [procedure-fixed-arity? (-> procedure? boolean?)]
+ (contract-out [procedure-fixed-arity? (-> procedure? boolean?)]
                [in-random (case-> (-> (stream/c (and/c real? inexact? (>/c 0) (</c 1))))
                                   (-> (integer-in 1 4294967087) (stream/c exact-nonnegative-integer?))
                                   (-> exact-integer? (integer-in 1 4294967087)
@@ -492,26 +500,6 @@
         (cons/c key-contract val-contract)))
 
 
-;;; ======================================
-;;; Additional list and hash map utilities
-;;; ======================================
-
-;;; Given a list of lists of the same length, transposes them.
-;;;
-;;; > (lists-transpose '((1 2) (a b)))
-;;; '((1 a) (2 b))
-;;;
-;;; This function is essentially in-parallel, wrapped in a couple
-;;; conversions.
-(define lists-transpose
-  (compose sequence->list
-           in-values-sequence
-           ((curry apply) in-parallel)))
-
-(module+ test
-  (test-case "lists-transpose"
-    (check-equal? (lists-transpose '((1 2) (a b))) '((1 a) (2 b)))))
-
 ;;; =========
 ;;; Functions
 ;;; =========