diff options
Diffstat (limited to 'libraries')
| -rw-r--r-- | libraries/base/Data/OldList.hs | 8 |
1 files changed, 7 insertions, 1 deletions
diff --git a/libraries/base/Data/OldList.hs b/libraries/base/Data/OldList.hs index 0252ae0fd8..9331113d70 100644 --- a/libraries/base/Data/OldList.hs +++ b/libraries/base/Data/OldList.hs @@ -550,7 +550,13 @@ intercalate xs xss = concat (intersperse xs xss) transpose :: [[a]] -> [[a]] transpose [] = [] transpose ([] : xss) = transpose xss -transpose ((x:xs) : xss) = (x : [h | (h:_) <- xss]) : transpose (xs : [ t | (_:t) <- xss]) +transpose ((x:xs) : xss) = (x : hds) : transpose (xs : tls) + where + -- We tie the calculations of heads and tails together + -- to prevent heads from leaking into tails and vice versa. + -- unzip makes the selector thunk arrangements we need to + -- ensure everything gets cleaned up properly. + (hds, tls) = unzip [(hd, tl) | (hd:tl) <- xss] -- | The 'partition' function takes a predicate a list and returns |