[Haskell-cafe] exporting Constructors as pattern-only

[Andrew Butterfield]

All,

 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.

 e.g

given

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) ?


Regards,

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