[Haskell] standard monads and strictness

[Wolfgang Jeltsch]

Hello,

I looked at the implementation of Writer, WriterT, State, StateT, RWS and 
RWST.  They all use tuples to knit the result with the written value and/or 
state.

Now, there seems to be an inconsistency between the transformer and 
non-transformer variants concerning strictness.  The non-transformer variants 
(Writer, State and RWS) use let binding to extract the tuple components while 
the transformer variants use pattern matching in do expressions.  So the 
expression

    evalState (undefined >> return ()) ()

yields () while

    evalStateT (undefined >> return ()) ()

yields _|_ for any inner monad.  Is there any good reason for this behaviour?

Since the tuples are only used for knitting together several values, the most 
logical solution would be to use unboxed tuples in all of the above cases, in 
my opinion.  If this is not feasible, we probably should use lazy pattern 
matching with the transformer versions.

Best wishes,
Wolfgang