Check out TypeError from GHC.TypeLits, then make a type family or class that resolves to TypeError when both params are identical. e.g. {-# LANGUAGE TypeInType, MultiParamTypeClasses, FlexibleInstances, FlexibleContexts #-} class CheckDistinct a b instance {-# OVERLAPS #-} TypeError (Text "Parameters must be distinct") => CheckDistinct a a instance CheckDistinct a b
:t undefined :: CheckDistinct Int Double => Int undefined :: CheckDistinct Int Double => Int :: Int
:t undefined :: CheckDistinct Int Int => Int <interactive>:1:1: error: Parameters must be distinct
On 2017-08-21 06:52, Timotej Tomandl wrote:
As the title says.
Motivation: in a mutable code I want to keep some parameters strictly immutable, i.e. not sharing s with the whole mutable type
I think this is best illustrated by 2 examples: 1. abstract example: imagine I have a type X a b c and another type Y a b and I want to prevent any case where the first parameter of X "a" is shared with it's third parameter "c" i.e. X a b (Y a a), X a b (Y a b), X a b (Y b a), X a b (Y a d) etc., any nestings in Y referencing a etc. but allow X a b (Y b b), X a b (Y b c) and any other combination not ruled out above 2. concrete example: imagine I have a type: Item s element key where s is used in the same way as in STRef s a, i.e. it cannot leak outside of a certain context, but I want the key to be immutable i.e. independent of s under any circumstences, so I can't write a type like: Item s element (STRef s refType) this is equivalent to disallowing X a b (Y a d) in 1.
Is there any way to write a restriction like this in Haskell?
-- Timo
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