How do you extract the first element from a Maybe tuple? I have tried to use fst but that doesn't seem to work.
2 Answers
Since Maybe is a functor, use fmap to lift fst :: (a, b) -> a to work with Maybe (a,b).
> :t fmap fst fmap fst :: Functor f => f (b, b1) -> f b > fmap fst $ Just (3, 6) Just 3 > fmap fst $ Nothing Nothing Of course, this returns a Maybe a, not an a, so you can use the maybe function to unpack the result (and provide a default value if the Maybe (a, b) is actually Nothing):
> import Data.Maybe > maybe 0 fst (Just (3, 6)) 3 > maybe 0 fst Nothing 0 
Comments
You can pattern match on Maybe value using case, for example:
case mbVal of Just x -> fst x Nothing -> ... You can also use fromJust if you are sure the value is Just.
Finally, you can match the first element of a tuple right away:
case mbVal of Just (x,_) -> x 
Maybevalue contains aNothing)?maybe undefined fst, or(fst <$>).