Unifying record type

Question

As a learning exercise, I'm trying to define a newtype to serve as a holder of functions that can convert Show-able values to Effects, i.e.:

newtype ShEff a = ShEff (forall eff. Show a => a -> Eff eff Unit)

However, this:

f :: forall a. ShEff a
f = ShEff logShow

fails to compile with this error:

  Could not match type

    ( console :: CONSOLE
    | t2
    )

  with type

    eff1


while trying to match type Eff
                             ( console :: CONSOLE
                             | t2
                             )
  with type Eff eff1
while checking that expression logShow
  has type t0 -> Eff eff1 Unit
in value declaration f

Can you point me in the right direction?

Phil Freeman Phil Freeman · Accepted Answer · 2017-04-07T02:18:31

The type of logShow is

forall eff. Show a => a -> Eff (console :: CONSOLE | eff) Unit

so you can't store it inside ShEff, since that must work for every eff, and logShow only works for rows containing the CONSOLE effect.

You have two options:

Move the eff type argument outside ShEff:

newtype ShEff eff a = ShEff (a -> Eff eff Unit)

f :: forall a eff. Show a => ShEff (console :: CONSOLE | eff) a
f = ShEff logShow

Add the constraint inside ShEff:

newtype ShEff a = ShEff (forall eff. a -> Eff (console :: CONSOLE | eff) Unit)

f :: forall a eff. Show a => ShEff a
f = ShEff logShow

Also notice that I've moved the Show a constraint outside ShEff in both cases.

Unifying record type

1 Answers