Wow... my bad. Stream is in no way a monad transformer.
I really should read before speaking...

Stream s m t is such as "An instance of Stream has stream type s, underlying monad m and token type t determined by the stream" (http://hackage.haskell.org/packages/archive/parsec/3.1.1/doc/html/Text-Parsec-Prim.html#t:Stream)

ParsecT is the monad transformer, which when stacked with Identity gives a simple monad, aliased by the type Parsec :
type Parsec s u = ParsecT s u Identity


2011/10/8 Yves Parès <limestrael@gmail.com>
If I have this right, Stream is a monad transformer.
Stream s m t means that it parses 's', is stacked with monad 'm' and has a result of type 't'

So Identity is a monad, the simplest monad, defined as such:

newtype Indentity t = Identity { runIdentity :: t }

It's the identity monad, that does nothing special but wrapping its result.


2011/10/8 Captain Freako <capn.freako@gmail.com>
Hi all,

In this definition from the Parsec library:

parse :: (Stream s Identity t)
      => Parsec s () a -> SourceName -> s -> Either ParseError a
parse p = runP p ()
what's the significance of `Identity t'?
(`t' isn't used anywhere.)

Thanks,
-db


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