[Haskell-beginners] fmap versus bind

[Patrick LeBoutillier]

Felipe,

>
> Regardless, there's a trick that you may use whenever you want to know
> the types of something.  Just make a function where everything you
> don't know is an argument, and ask GHCi to give you its type.

That's a great trick.

Thanks everybody for your help.


Patrick

> Supposing it's Haskell 98, GHCi will always give you the most general
> type.  With this example:
>
> Prelude> :t \ds applyOp op x y -> applyOp op x y >>= flip (:) ds
> \ds applyOp op x y -> applyOp op x y >>= flip (:) ds
>  :: [a] -> (t -> t1 -> t2 -> [a]) -> t -> t1 -> t2 -> [a]
>
> So we see that
>
>  ds :: [a]
>  applyOp :: t -> u -> v -> [a]
>  op :: t
>  x :: u
>  y :: v
>  applyOp op x y >>= flip (:) ds :: [a]
>
> works.  In other words, it's an expression in the list monad.  Whether
> it is useful or not, I don't know =).  Adding the 'return':
>
> Prelude> :t \ds applyOp op x y -> applyOp op x y >>= return . flip (:) ds
> \ds applyOp op x y -> applyOp op x y >>= return . flip (:) ds
>  :: (Monad m) =>
>     [a] -> (t -> t1 -> t2 -> m a) -> t -> t1 -> t2 -> m [a]
>
> So now we get:
>
>  Monad m =>
>  ds :: [a]
>  applyOp :: t -> u -> v -> m a
>  op :: t
>  x :: u
>  y :: v
>  applyOp op x y >>= return . flip (:) ds :: m [a]
>
> Seems much more useful!  So 'applyOp op x y :: m a' and the result of
> everything is 'm [a]'.
>
> Note that if there is a type mismatch (even with the general
> arguments!), GHCi will tell you right away.  For example,
>
> Prelude> :t \ds applyOp op x y -> applyOp op x y >>= fmap . flip (:) ds
>
> <interactive>:1:48:
>    Couldn't match expected type `a -> b' against inferred type `[a1]'
>    In the second argument of `(.)', namely `flip (:) ds'
>    In the second argument of `(>>=)', namely `fmap . flip (:) ds'
>    In the expression: applyOp op x y >>= fmap . flip (:) ds
>
> This means that no matter what ds, applyOp, op, x and y you choose,
> 'applyOp op x y >>= fmap . flip (:) ds' will always be ill-typed.
>
> Cheers,
>
> --
> Felipe.
>



-- 
=====================
Patrick LeBoutillier
Rosemère, Québec, Canada