[Haskell-cafe] exporting Constructors as pattern-only

[Andrew Butterfield]

Hi Jon, 
thanks for that speedy response - looks ideal!

I guess though that any tests for module Tee will have to be written inside Tee,
rather than in a separate test file - again this is no big deal.

Regards, Andrew

> On 19 May 2017, at 10:10, Jon Purdy <evincarofautumn at gmail.com> wrote:
> 
> 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 at scss.tcd.ie <mailto:Andrew.Butterfield at 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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Andrew Butterfield
School of Computer Science & Statistics
Trinity College
Dublin 2, Ireland

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