[Haskell-cafe] MultiCase alternative
Richard A. O'Keefe
ok at cs.otago.ac.nz
Thu Jun 15 23:31:32 UTC 2017
There is another elementary alternative. If you need to treat C and
D the same in just one place, you don't really have a problem. If
you need to treat them the same in several places, do this:
data T a b c = A a | B b | C c | D c -- existing type
data Which a b c = A' a | B' b | CD Bool c
which :: T a b c -> Which a b c
which (A a) = A' a
which (B b) = B' b
which (C c) = CD False c
which (D c) = CD True c
case which $ x of
A' a ->
B' b ->
CD _ c -> ...
If you want to merge the C and D cases often, I like this approach,
C c -> f c
D c -> f c
where f c = ...
approach is better.
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