'No type class instance was found' on multi-parameter type class

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Is it reasonable to have multi parameter type classes in PureScript?

The compiler raises an unreasonable 'No type class instance was found' error (see full output) for the below type class definition:

class Transform model turn where
  transform :: turn -> model -> model
  delay :: Maybe turn -> Int

The error arises from the definition of the delay function which doesn't depend on the model type parameter. As evident from the compiler output linked above it substitutes a t0 for the absent model.

What is the proper way to fix this? Currently I workaround this issue by changing the kind of turn as below:

class Turnable model turn where
  runTurn :: turn model -> model -> model
  turnDelay :: Maybe (turn model) -> Int
2

There are 2 answers

0
Sridhar Ratnakumar On

rightfold from the FP slack channel gave this answer:

add fundep | turn -> model

Then the instance is looked up only through turn

Consequently I changed the type class definition to this:

class Turnable model turn | turn -> model where
  runTurn :: turn -> model -> model
  turnDelay :: Maybe turn -> Int

This made the compiler happy. For details, consult Functional dependencies (fundep) in the Haskell wiki and the 24 days of PureScript post.

0
pete On

...in the old days we had used a Proxy (https://github.com/purescript/purescript-proxy) to help the compiler decide which instance to choose. e.g. :

turnDelay :: Proxy model -> turn -> Int
turnDelay _ turn = .....

and call it by passing in a proxy:

turnDelay (Proxy :: Proxy FooModel) barTurn

I agree FunDeps are the right way to go.