diff options
| -rw-r--r-- | compiler/utils/Util.hs | 29 |
1 files changed, 17 insertions, 12 deletions
diff --git a/compiler/utils/Util.hs b/compiler/utils/Util.hs index 876cd1ee6e..16864fe017 100644 --- a/compiler/utils/Util.hs +++ b/compiler/utils/Util.hs @@ -783,20 +783,25 @@ lastMaybe :: [a] -> Maybe a lastMaybe [] = Nothing lastMaybe xs = Just $ last xs --- | If there is a good chance that you will only look at the last --- element prefer seperate calls to @last@ + @init@. --- @last@ does not allocate while traversing the list, while this --- will. But if you are guaranteed to use both this will --- usually be more efficient. +-- | Split a list into its last element and the initial part of the list. +-- @snocView xs = Just (init xs, last xs)@ for non-empty lists. +-- @snocView xs = Nothing@ otherwise. +-- Unless both parts of the result are guaranteed to be used +-- prefer separate calls to @last@ + @init@. +-- If you are guaranteed to use both, this will +-- be more efficient. snocView :: [a] -> Maybe ([a],a) - -- Split off the last element snocView [] = Nothing -snocView xs = go [] xs - where - -- Invariant: second arg is non-empty - go acc [x] = Just (reverse acc, x) - go acc (x:xs) = go (x:acc) xs - go _ [] = panic "Util: snocView" +snocView xs + | (xs,x) <- go xs + = Just (xs,x) + where + go :: [a] -> ([a],a) + go [x] = ([],x) + go (x:xs) + | !(xs',x') <- go xs + = (x:xs', x') + go [] = error "impossible" split :: Char -> String -> [String] split c s = case rest of |