[Haskell-cafe] Partially applied type synonyms

[Ttt Mmm]

>     On 10/08/2021 5:33 AM Anthony Clayden <anthony.d.clayden at gmail.com> wrote:
> 
> 
> 
>     > I was surprised to find the below code doesn't typecheck even with -XLiberalTypeSynonyms. Am I missing something or is this really not possible?
> 
> 
>     Reading the error message would help:
> 
>     * The type synonym `Foo' should have 2 arguments, but has been given 1
> 

I'm not sure what in my question made it seem like I hadn't read this error message. With my code as written I can't apply both arguments, and I'm trying to solve the problem in front of me given that fact.


>     -XLiberalTypeSynonyms relaxes the 'saturated application' rule a little https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/liberal_type_synonyms.html?highlight=type%20synonym#liberalised-type-synonyms
> 
>     "You can apply a type synonym to a partially applied type synonym:"
> 
>     But that isn't what you're doing. The docs say that still
> 
>     " ... malformedness ... * Partially-applied type synonym."
> 
>     Type Synonyms are not functions. Think of them as 'macro' expansions.
> 

Interestingly I get an almost identical error when defining Foo as a type family, which seems a lot more akin to a function.


>     Possibly your `Bar` could be a type synonym, taking `(Foo Maybe)` as an argument in the `instance Eq`.
> 

Thank you for this concrete suggestion. I'd actually tried variations on this before asking on the list, and I've tried a few more just now, but I'm very willing to believe I'm simply not looking in the right place. Certainly a simple `type Bar' x = Bar x` isn't changing anything.


>     What type is it exactly that you want to derive an instance for?
> 

Other than excluding unnecessary details, I've provided code that's pretty similar to the real code I'm working on. The biggest difference is that `Bar` is actually defined more like:

data Bar (m :: * -> *) = Bar { a :: m Int, b :: m Bool, c :: m Double }


So it really does need to take a `* -> *` type

Thanks,
Tom

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