1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
|
{-# LANGUAGE NoImplicitPrelude #-}
module Data.Maybe where
import Data.Bool
data Maybe a = Just a | Nothing
maybe n _ Nothing = n
maybe _ f (Just x) = f x
isJust Nothing = False
isJust _ = True
isNothing Nothing = True
isNothing _ = False
fromJust (Just x) = x
-- fromJust Nothing = error "Maybe.fromJust: Nothing"
fromMaybe d x = case x of Nothing -> d
Just v -> v
maybeToList Nothing = []
maybeToList (Just x) = [x]
listToMaybe [] = Nothing
listToMaybe (x:_) = Just x
-- listToMaybe = foldr (const . Just) Nothing
-- GHC uses this to fused via the foldr/build rule.
-- catMaybes ls = [ x | Just x <- ls]
mapMaybes _ [] = []
mapMaybes f (x:xs) = let rs = mapMaybes f xs
in case f x of Nothing -> rs
Just r -> r:rs
|
