Drop the redundant r from the definition of Parser

[nikita-volkov]

Currently it's impossible to use Parser in a monad-transformer stack. In cases like the following:

type MyParser s b m a = StateT s (Pipes.Parse.Parser b m) a

the compiler complains with:

Type synonym ‘Pipes.Parse.Parser’ should have 3 arguments, but has been given 2

So I suggest to remove the redundant r from the definition of Parser:

- type Parser a m r = forall x . StateT (Producer a m x) m r
+ type Parser a m = forall x . StateT (Producer a m x) m

Of course, it's possible to work around the issue by expanding the type synonym in place, but it's still an issue.

[Gabriella439]

This doesn't work (the library won't compile if you delete the r on both sides), because now you're universally quantifying a higher-kinded type, which is almost always not going to behave well.

The reason this causes problems is that if you write:

Parser a m r

... the compiler will expand it to:

(forall x . StateT (Producer a m x) m) r

... which is not the same thing as:

forall x . StateT (Producer a m x) m r

[nikita-volkov]

Sorry. My bad.

[Gabriella439]

No problem at all! :)