[Haskell-beginners] More Data.Vector fun

[Philip Scott]

Hey ho,

This should be a quick one, I hope! I am mucking about with the moadic 
initialisers for vectors, and am experiencing all kinds of fun. For 
example, I think I should be able to make a new vector of, say, 3 
thingies* long:

Prelude Data.Vector.Generic.Mutable> new 3

Now of course, poor old haskell does not know what sort of thing I am 
going to put in it yet, so it gets a bit cross:

<interactive>:1:0:
     Ambiguous type variable `m' in the constraint:
       `Control.Monad.Primitive.PrimMonad m'
         arising from a use of `new' at <interactive>:1:0-4
     Probable fix: add a type signature that fixes these type variable(s)

<interactive>:1:0:
     Ambiguous type variables `v', `a' in the constraint:
       `MVector v a' arising from a use of `new' at <interactive>:1:0-4
     Probable fix: add a type signature that fixes these type variable(s)


Now I have paid very close attention to this 'Probable Fix', but have 
not been able to find out precisely how to specify what sort of vector I 
would like (I guess this is v..) and am even more boggled by 'm'.. I 
guess this is asking what sort of Monad I am going to use to fill it up? 
An example would be greatly appreciated.

I find the type of 'new' quite interesting:

new
   :: forall (m :: * -> *) (v :: * -> * -> *) a.
      (PrimMonad m, Data.Vector.Generic.Mutable.MVector v a) =>
      Int -> m (v (PrimState m) a)

So I did a bit of reading up on exstential types, and I (just about) get 
what 'forall' does, however I don't know what the (m :: * -> *) and  (v 
:: * -> * -> *) bits mean. Just telling me what they are called would 
probably be enough to get me googling!

I get the feeling I am a bit out of my depth here, but hey ho, I guess 
that's how you learn to swim!

All the best,

Phil

* thingies == things of some type, which doesn't really matter at the 
momant.