Wed, 16 Jul 2003 14:01:36 +0200
Sorry if this mail starts a new thread. I am not subscribed to haskell-cafe
and am new to hotmail.
Uhm, as far as the example goes. I was trying to define a small (shallow
encoding of) a reactive systems language. Because I wanted to try something
else than monads I defined the following recursive type for a reactive
type R m = m -> Maybe (R m, [m])
Supposedly a reactive system is a system which may not take a message (the
Maybe), or do take it and respond with an updated version of itself, and a
sequence of outgoing messages.
For example, a copying machine might be defined as:
id m = Just (id, [m])
A version for a ?Meally? machine embedding (always take an incoming message,
and respond with one outgoing message) could be written as
type Meally i o = i -> (Meally i o, o)
where id is defined as
id::Meally a a
id m = (id, m)
I like the formalization because (a) it is concise, (b) structural
composition operators can be defined trivially, (c) hides the state of a
machine, (d) might possibly, in the long run, somewhere, even work out
equally well as a monadic approach.
Does it make sense? Are there any known references to this approach?
PS: Actually, I just realized this might be the co-algebraic approach, so I
guess I am asking for a haskell which allows co-algebraic types.
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