GHC Error Message.

Sun, 09 Mar 2003 03:45:40 -0800

Hi,
 
Could someone please explain to me why the following error message happen:
 
When I load the following code into GHC I get an error message:
 
code:
---------------------------------------------------------------------------------------------------
data AParser String = AP {apapply::([String]->[(String,[String])])}
 
instance Monad AParser where
  return v        = AP (\inp -> [(v,inp)])
  (AP p) >>= f    = AP (\inp -> concat [ apapply (f v) inp1 | (v,inp1) <- p inp])
---------------------------------------------------------------------------------------------------
 
error:
---------------------------------------------------------------------------------------------------
    Inferred type is less polymorphic than expected
        Quantified type variable `b' is unified with another quantified type variable `a'
    When trying to generalise the type inferred for `>>='
        Signature type:     forall a1 b1.
                            AParser a1 -> (a1 -> AParser b1) -> AParser b1
        Type to generalise: forall a1 b1.
                            AParser a1 -> (a1 -> AParser b1) -> AParser b1
    In the instance declaration for `Monad AParser'
Failed, modules loaded: none.
---------------------------------------------------------------------------------------------------
 
But when I create the type synonym
type Stack = [String]
data AParser String = AP {apapply::(Stack -> [(String,Stack)])}
 
and use Stack instead of [String], GHC does not complain???
 
Thanks.
Mansour.

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