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replacing <*> with ap for maybe results in an exception #189

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luminusian opened this issue Feb 22, 2017 · 0 comments

luminusian commented Feb 22, 2017

(<*>) = ap, although more efficient implementations may exist
https://wiki.haskell.org/Functor-Applicative-Monad_Proposal#Compatibility_issues

However,

(m/<*> (m/<$> (m/curry 2 +) (maybe/just 1)) (maybe/just 2))
=> #<Just 3>
(m/ap (m/<$> (m/curry 2 +) (maybe/just 1)) (maybe/just 2))
CompilerException java.lang.ClassCastException: cats.monad.maybe.Just cannot be cast to clojure.lang.IFn, compiling:

ap                :: (Monad m) => m (a -> b) -> m a -> m b
ap m1 m2          = do { x1 <- m1; x2 <- m2; return (x1 x2) }

https://hackage.haskell.org/package/base-4.9.1.0/docs/src/GHC.Base.html#ap

(defn ap
  [m1 m2]
  (m/mlet [x1 m1
           x2 m2]
          (m/return (x1 x2))))

(ap (m/<$> (m/curry 2 +) (maybe/just 1)) (maybe/just 2))
=> #<Just 3>

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