***DHSPAM*** Re: [Haskell-cafe] Newbie Q: Monad 'fail' and 'error'

[Juan Carlos Arevalo Baeza]

On Wed, 06 Jun 2007 09:29:09 -0700, Dmitri O.Kondratiev  
<dokondr at gmail.com> wrote:

> Default 'fail' implementation in Monad class will be:
> *DivBy> divBy 5 0
> Loading package haskell98-1.0 ... linking ... done.
> *** Exception: user error (div by zero)
>
> And when explicitly defining monad as Maybe it will be different:
> *DivBy> divBy 5 0::Maybe Int
> Nothing
>
> I am curious if it is possible to  'cast' divBy to List, Identity, other
> monads? How?

    It just works, and you already did it for Maybe. Just provide the type:

*Main> divBy 5 0::[Int]
[]
*Main> divBy 5 1::[Int]
[5]

    Identity fails with a stack overflow, though (GHC 6.6):

*Main> divBy 5 0::Identity Int
*** Exception: stack overflow
*Main> fail "KK" :: Identity ()
*** Exception: stack overflow
*Main> divBy 5 1::Identity Int
*** Exception: stack overflow

    Anyway...

JCAB