[Haskell-cafe] Client-extensible heterogeneous types

Evan Laforge qdunkan at gmail.com
Wed Oct 13 17:52:14 EDT 2010

I admit I haven't read this whole thread in detail, but when I want
something with an implementation that can vary dynamically I just pass
a different function.  Your original python example is equivalent to
just passing strings in haskell, so lets add an argument:

type Process = Int -> String

heterogeneousProcessor :: [Process] -> [String]
heterogeneousProcessor ps = [p 42 | p <- ps] -- or map ($42) ps

variant1 n = "variant1 stuff " ++ show n
-- etc.

Now the user of your library can pass their own Process.

I have a number of records in my program like "State { lookup_x ::
Name -> Maybe X, lookup_y :: Name -> Maybe Y, do_something_important
:: X -> Result }".  They reduce dependencies by not exposing the
(complicated) lookup details and types, and aid testing because I can
just pass a state with a dummy 'do_something_important' (in my case,
it's "update GUI", which is important to stub out for a test).

This may be simpler than what you had in mind, but to narrow it down,
could you provide a more specific example where this is inadequate?

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