[Haskell-cafe] an idea for modifiyng data/newtype syntax: use `::=` instead of `=`

MigMit miguelimo38 at yandex.ru
Sat Aug 8 22:16:27 UTC 2015

>> 1) This might be the code written by someone using your library/framework.
>> In which case it would know about A and B.
> Then it's up to me to define and document whatever strictness properties I
> want for my constructors.

Of course. But it's NOT up to you to restrict the user from using whatever techniques xe wants

> in existing code.  Of course that won't work.  But for *new* datatypes
> choosing one rather than the other gives no difference in terms of
> denotational semantics.

If it makes the difference for the old code, then it would make the difference for the new code as well.

>> 2) It might be generated by the Template Haskell — which is free to use
>> whatever constructor is fed into it.
> OK, so show me what goes wrong!

I hate it.

OK, here we go:

{-# LANGUAGE TemplateHaskell #-}
module TH where
import Language.Haskell.TH
check :: Name -> ExpQ
check c = [|let x = case x of $(conP c [[p|_|]]) -> $(conE c) 1 in x|]

{-# LANGUAGE TemplateHaskell #-}
module Use where
import TH
newtype A = A Int deriving Show
data B = B !Int deriving Show
a = $(check 'A)
b = $(check 'B)

Prelude> :load "Use.hs"
[1 of 2] Compiling TH               ( TH.hs, interpreted )
[2 of 2] Compiling Use              ( Use.hs, interpreted )

Use.hs:6:1: Warning:
    Top-level binding with no type signature: a :: A

Use.hs:7:1: Warning:
    Top-level binding with no type signature: b :: B
Ok, modules loaded: TH, Use.
*Use> a
A 1
*Use> b
  C-c C-cInterrupted.

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