[Haskell-beginners] ap = liftM2 id

[Rein Henrichs]

Following up on Andres's explanation, we have

ap f x = liftM2 id mf ma                          -- definition of ap
       = do { f <- mf; a <- ma; return (id f x) } -- definition of liftM2
       = do { f <- mf; a <- ma; return (f x) }    -- definition of id, f x
y = (f x) y

We can see that id applied to f gives f which is then applied to a.
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