[Haskell-cafe] forall vs "=>"
jason.dusek at gmail.com
Wed May 13 23:45:54 EDT 2009
2009/05/12 Daryoush Mehrtash <dmehrtash at gmail.com>:
> runST :: (forall s. ST s a) -> a
The `forall` here has a rather elaborate purpose.
> evalStateT :: Monad m => StateT s m a -> s -> m a
"Where `m` is in `Monad`, we have...
Let's put in the "hidden `forall`":
evalStateT :: forall s m a. (Monad m) => StateT s m a -> s -> m a
The `(Monad m)` is our "class context" -- it tells us that `m`
must have an implementation for `Monad`. The `forall` is
simple quantification (type signatures are implicitly
universally quantified). Looking at your first example, let's
put in the hidden `forall`:
runST :: forall a. (forall s. ST s a) -> a
This is a little fancy. It tells us that for a given `a`,
`runST` must accept a stateful computation with a state of any
type whatsoever; hence we can not return the type of the state
as the result of the computation. My explanation makes short
work of an interesting topic; using class contexts is quite a
bit more common than using tricky `forall`s.
The important thing to seed is that the `=>` introduces a
class constraint/condition on our type signature. You can
imagine that it is always there:
id :: forall a. () => a -> a
You can read `=>` as "entails". Then `() =>` is "the universe
entails..." and `(Monad m) =>` is "the universe with `m` in
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