[Haskell-cafe] exporting Constructors as pattern-only

Andrew Butterfield Andrew.Butterfield at scss.tcd.ie
Fri May 19 08:43:56 UTC 2017


 is there any way in Haskell to export a *data* type so that importing modules can pattern match,
but not use the constructors to build anything?

My use case is an AST with invariant - I want the convenience of pattern matching
with the safety of having to build using functions exported by the model rather than the constructors directly.



data T = T1 Bool | T2 Int | TT T T
t1 :: Bool -> T
t2 :: Int -> T
tt :: T -> T -> T

from outside I can write

f(T1 False) =  tt (t1 True) (t2 42)

but not

f(T1 False) = TT (T1 True) (T2 42) ?


Andrew Butterfield
School of Computer Science & Statistics
Trinity College
Dublin 2, Ireland

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