Re: [Haskell-cafe] Intro to monad transformers

26 Dec 2010

      Ok, that works. On to code reading and, hopefully, enlightenment.

Thanks, all.

Michael

--- On Sun, 12/26/10, Daniel Fischer  wrote:

From: Daniel Fischer 
Subject: Re: [Haskell-cafe] Intro to monad transformers
To: haskell-cafe@haskell.org
Cc: "michael rice" , "David Menendez" 
Date: Sunday, December 26, 2010, 4:07 PM

On Sunday 26 December 2010 21:21:00, michael rice wrote:
...
Ok, changed the last line and deleted the bad line. Maybe someone could
recommend a better example?
Michael
=============
Prelude> :l test5
[1 of 1] Compiling Main             ( test5.hs, interpreted )
test5.hs:16:4:
    Occurs check: cannot construct the infinite type: a = Maybe a
    When generalising the type(s) for `mplus'
    In the instance declaration for `MonadPlus (MaybeT m)'
Failed, modules loaded: none.

...
    mplus x y = MaybeT $ do maybe_value <- runMaybeT x
                            case maybe_value
 of
                                 Nothing    -> runMaybeT y
                                 Just value -> runMaybeT x
The last line is wrong. It should be, "Just value -> return value".
Actually, it should be

    case maybe_value of
      Nothing -> runMaybeT y
      _ -> return maybe_value