問題

PureScript で Maybe a を a にしたい (値を取り出したい) 。どうすれば良いのか。

解決策

あくまで一例。

case などのパターンマッチで取り出す。
maybe (https://pursuit.purescript.org/packages/purescript-maybe/2.0.1/docs/Data.Maybe#v:maybe) を使う。
fromMaybe (https://pursuit.purescript.org/packages/purescript-maybe/2.0.1/docs/Data.Maybe#v:fromMaybe) を使う。
fromJust (https://pursuit.purescript.org/packages/purescript-maybe/2.0.1/docs/Data.Maybe#v:fromJust) + unsafePartial を使う。

基本的には値がない (Nothing) ときの代わりの値については考えないといけない。次の例では 456 を使っている。

fromJust は Nothing のパターンを考慮していない部分関数。 unsafePartial を使って、危険を承知で Just 前提で抜き出す。

`case`

> import Data.Maybe (Maybe(..))
> let m1 = Just 123
> let m2 = Nothing
>
> :paste
… case m1 of
…   Nothing -> 456
…   Just x -> x
…
123

> :paste
… case m2 of
…   Nothing -> 456
…   Just x -> x
…
456

`maybe`

> import Data.Maybe (Maybe(..), maybe)
> let m1 = Just 123
> let m2 = Nothing
>
> maybe 456 (\x -> x) m1
123

> maybe 456 (\x -> x) m2
456

`fromMaybe`

> import Data.Maybe (Maybe(..), fromMaybe)
> let m1 = Just 123
> let m2 = Nothing
>
> fromMaybe 456 m1
123

> fromMaybe 456 m2
456

`fromJust` (+ `unsafePartial`)

> import Data.Maybe (Maybe(..), isJust, fromJust)
> import Partial.Unsafe (unsafePartial)
> let m1 = Just 123
> let m2 = Nothing
>
> if isJust m1 then unsafePartial fromJust m1 else 456
123

> if isJust m2 then unsafePartial fromJust m2 else 456
456

> unsafePartial fromJust m1
123

> unsafePartial fromJust m2 :: Maybe Int
...
Error: Failed pattern match
...

実装

ちなみにそれぞれの実装は次の通り。

maybe https://github.com/purescript/purescript-maybe/blob/v2.0.1/src/Data/Maybe.purs#L213-L213

maybe :: forall a b. b -> (a -> b) -> Maybe a -> b
maybe b _ Nothing = b
maybe _ f (Just a) = f a

fromMaybe https://github.com/purescript/purescript-maybe/blob/v2.0.1/src/Data/Maybe.purs#L238-L238

fromMaybe :: forall a. a -> Maybe a -> a
fromMaybe a = maybe a id

fromJust https://github.com/purescript/purescript-maybe/blob/v2.0.1/src/Data/Maybe.purs#L264-L265

fromJust :: forall a. Partial => Maybe a -> a
fromJust (Just x) = x

参考

https://pursuit.purescript.org/packages/purescript-maybe/2.0.1/docs/Data.Maybe#v:maybe
https://pursuit.purescript.org/packages/purescript-maybe/2.0.1/docs/Data.Maybe#v:fromMaybe
https://github.com/purescript/purescript-maybe/blob/v2.0.1/src/Data/Maybe.purs#L213-L213
https://github.com/purescript/purescript-maybe/blob/v2.0.1/src/Data/Maybe.purs#L238-L238
https://github.com/purescript/purescript-maybe/blob/v2.0.1/src/Data/Maybe.purs#L264-L265