[Haskell-cafe] collection monads

tpledger at ihug.co.nz tpledger at ihug.co.nz
Sun Oct 8 05:57:46 EDT 2006

Matthias Fischmann wrote:
> > Do you expect the contained type x to change during a
> > sequence of monadic actions?  e.g. would you ever use
> > at the type 'Permutation Int -> (Int -> Permutation
Bool) ->
> > Permutation Bool'?
> no, i don't need that.  but aside from
> the fact that
> >     data Permutation k v =
> >         Permutation [(k, v)]
> >     instance (Ix k) =>
> >         Monad (Permutation k)
> is redundant (i think of the permutation
> as a function applicable to arbitrary
> lists): how would that change anything?
> my definition of return still doesn't
> work.  or how would you redefine
> 'return'?

Ah.  Yes, my approach falls over because it lacks two
things.  #1: a distinguished value of the Ix-constrained
type k, to pair off with return's argument.  #2: a purpose. 
I don't have a clear idea of what a do-block in a
permutation monad ought to mean.  Whoops!  <font

Tom (crawling back under his rock)

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