[Haskell-cafe] Partially applied type synonyms

Anthony Clayden anthony.d.clayden at gmail.com
Fri Oct 8 03:33:07 UTC 2021

> I was surprised to find the below code doesn't typecheck even with
-XLiberalTypeSynonyms. Am I missing something or is this really not

Reading the error message would help:

* The type synonym `Foo' should have 2 arguments, but has been given 1

-XLiberalTypeSynonyms relaxes the 'saturated application' rule a little

"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.

Possibly your `Bar` could be a type synonym, taking `(Foo Maybe)` as an
argument in the `instance Eq`. What type is it exactly that you want to
derive an instance for?
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