diff options
Diffstat (limited to 'compiler/GHC/Data/Maybe.hs')
| -rw-r--r-- | compiler/GHC/Data/Maybe.hs | 12 |
1 files changed, 11 insertions, 1 deletions
diff --git a/compiler/GHC/Data/Maybe.hs b/compiler/GHC/Data/Maybe.hs index 230468a20e..ac9c687b62 100644 --- a/compiler/GHC/Data/Maybe.hs +++ b/compiler/GHC/Data/Maybe.hs @@ -16,7 +16,7 @@ module GHC.Data.Maybe ( failME, isSuccess, orElse, - firstJust, firstJusts, + firstJust, firstJusts, firstJustsM, whenIsJust, expectJust, rightToMaybe, @@ -31,6 +31,7 @@ import Control.Monad import Control.Monad.Trans.Maybe import Control.Exception (catch, SomeException(..)) import Data.Maybe +import Data.Foldable ( foldlM ) import GHC.Utils.Misc (HasCallStack) infixr 4 `orElse` @@ -51,6 +52,15 @@ firstJust a b = firstJusts [a, b] firstJusts :: [Maybe a] -> Maybe a firstJusts = msum +-- | Takes computations returnings @Maybes@; tries each one in order. +-- The first one to return a @Just@ wins. Returns @Nothing@ if all computations +-- return @Nothing@. +firstJustsM :: (Monad m, Foldable f) => f (m (Maybe a)) -> m (Maybe a) +firstJustsM = foldlM go Nothing where + go :: Monad m => Maybe a -> m (Maybe a) -> m (Maybe a) + go Nothing action = action + go result@(Just _) _action = return result + expectJust :: HasCallStack => String -> Maybe a -> a {-# INLINE expectJust #-} expectJust _ (Just x) = x |