I believe you can use unidirectional pattern synonyms, and only export the patterns, not the constructors.

{-# LANGUAGE PatternSynonyms #-}

module Tee (T, t1, t2, pattern T1, pattern T2) where

data T = MkT1 Bool | MkT2 Int | …

pattern T1 a <- MkT1 a

pattern T2 a <- MkT2 a

…

t1 :: Bool -> T

t2 :: Int -> T

…

You can pattern-match on T1 just fine, but if you try to use it as a constructor you’ll get “non-bidirectional pattern synonym ‘T1’ used in an expression”.

On Fri, May 19, 2017 at 1:43 AM, Andrew Butterfield <Andrew.Butterfield@scss.tcd.ie> wrote:

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

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