From 8868d00981ed87ac5d4c926c2a200cd08db32b4a Mon Sep 17 00:00:00 2001
From: Rogerio Chaves <rogeriocfj@gmail.com>
Date: Sun, 24 Sep 2017 20:57:04 -0300
Subject: [PATCH] make transpose call stack size safe

---
 src/List/Extra.elm | 13 +++++++++----
 tests/Tests.elm    |  5 +++++
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/src/List/Extra.elm b/src/List/Extra.elm
index 4cdfaa2..4c07c17 100644
--- a/src/List/Extra.elm
+++ b/src/List/Extra.elm
@@ -775,13 +775,18 @@ If some rows are shorter than the following rows, their elements are skipped:
 
 -}
 transpose : List (List a) -> List (List a)
-transpose ll =
+transpose =
+    transposeHelp []
+
+
+transposeHelp : List (List a) -> List (List a) -> List (List a)
+transposeHelp acc ll =
     case ll of
         [] ->
-            []
+            acc
 
         [] :: xss ->
-            transpose xss
+            transposeHelp acc xss
 
         (x :: xs) :: xss ->
             let
@@ -791,7 +796,7 @@ transpose ll =
                 tails =
                     filterMap tail xss
             in
-                (x :: heads) :: transpose (xs :: tails)
+                transposeHelp (acc ++ [ x :: heads ]) (xs :: tails)
 
 
 {-| Return the list of all subsequences of a list.
diff --git a/tests/Tests.elm b/tests/Tests.elm
index caa134f..921e864 100644
--- a/tests/Tests.elm
+++ b/tests/Tests.elm
@@ -123,6 +123,11 @@ all =
                     Expect.equal
                         (transpose [ [ 10, 11 ], [ 20 ], [], [ 30, 31, 32 ] ])
                         [ [ 10, 20, 30 ], [ 11, 31 ], [ 32 ] ]
+            , test "transposes large lists" <|
+                \() ->
+                    Expect.equal
+                        (transpose [ List.repeat 10000 1 ])
+                        (List.repeat 10000 [ 1 ])
             ]
         , describe "subsequences" <|
             [ test "computes subsequences" <|