Brandon S. Allbery KF8NH wrote:
I think the problem is that technically runST is a data constructor
(possibly not relevant)
No, at least not in GHC. It is a function.
which takes a function as a parameter (definitely relevant).
It takes the type (forall s. ST s a) as its only parameter.
How is that more or less a function than anything else?
In the normal compositional model, (f . g) x
= f (g x), you're conceptually invoking f on the result of g x (g is
independent of f); here, you're lifting the function g x into the ST
s a monad via f (g is dependent on f).
I don't think I am using any special lifting mechanism here.
The "return" function does exploit the fact that ST s has a Monad
instance, but I only used the "return" function for simplicity. The
same thing happens if you construct a function that explicitly
returns (forall s. ST s a) and use that instead of "return":
Prelude Control.Monad.ST> :set -fglasgow-exts
Prelude Control.Monad.ST> let {f :: a -> (forall s. ST s a); f x = return x}
Prelude Control.Monad.ST> runST (f 42)
42
Prelude Control.Monad.ST> (runST . f) 42
<interactive>:1:9:
Couldn't match expected type `forall s. ST s a'
against inferred type `ST s a1'
...
Here is another possible clue to what is happening. When I
try to define that same function f using monomorphic syntax,
it fails:
Prelude Control.Monad.ST> let {f :: a -> (forall s. ST s a); f = return}
<interactive>:1:39:
Inferred type is less polymorphic than expected
Quantified type variable `s' escapes
Expected type: a -> forall s1. ST s1 a
Inferred type: a -> ST s a
In the expression: return
In the definition of `f': f = return
(Of course, the MR is not relevant here, because I am providing
an explicit type signature.)
_ _ ... ... _ _
-Yitz
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