Re: [Haskell-cafe] Can type be changed along a >>= chain?

2009/10/12 Ketil Malde :

minh thu writes:

...

ghci> :t (>>=) (>>=) :: (Monad m) => m a -> (a -> m b) -> m b

This says that, you provide an a and you get a b. Nothing says the a and b have to be the same upon successive uses.

(But note that the monad 'm' has to be the same all the way. You can't switch from, say, IO to ST in the middle of the computation.)

...

Likewise,

Contrariwise, I'd say.

...

ghci> :t (+) (+) :: (Num a) => a -> a -> a

I.e. (+) requires the same type on both sides...

...

fromIntegral ((1 :: Int) + 2) + (3 :: Integer) 6

...but 'fromIntegral' converts it. Did I miss some subtle point here?

I was talking about the types of the two + applications. They have a different types, just like the >>= in the parent's message can have different types. My fromIntegral has a role similar to the read s :: Int of the original question. Cheers, Thu