Re: [Haskell-cafe] questions on lazy pattern, StateT monad

24 Nov 2005

      ...
From your example I can see "Lazy patterns are useful in contexts
where infinite data structures are being defined recursively" (quote
Hi Wolfgang,

Thanks for your response and examples! It helps a lot.

section 4.4 of Gentle Introduction to Haskell). But does it apply to
the mplus case? I mean the mplus in (mplus m1 m2) and the mplus in
(mplus m1' m2') are different due to the difference of Monads (one is
StateT s m, the other is just m). If I change the mplus inside lift to
something else like:

  mplus m1 m2       = do s <- peek
                         let m1' = runState s m1
                             m2' = runState s m2
                         ~(a,s') <- lift (other_func m1' m2')
                         poke s'
                         return a

Is it still required that (a,s') be lazy?

I just want to see how things works in an less obvious example like this one.

Thanks,
Fan

Re: [Haskell-cafe] questions on lazy pattern, StateT monad

Fan Wu