PureScript Type Error is hard to understand

Question

This compiles

foo ma = case ma of
  [Just a] -> newRVar 0 >>= a

view                :: forall a eff. 
                       M.Map String (Linker Number a eff) -> 
                       String -> 
                       Eff (reactive :: Reactive | eff) Unit
view         m yaml = case parseYAML yaml           of
  Right (View xs)  -> case (flip M.lookup) m <$> xs of
    (as)       -> foo as

This does not

foo ma = case ma of
  Just a -> newRVar 0 >>= a

view                :: forall a eff. 
                       M.Map String (Linker Number a eff) -> 
                       String -> 
                       Eff (reactive :: Reactive | eff) Unit
view         m yaml = case parseYAML yaml           of
  Right (View xs)  -> case (flip M.lookup) m <$> xs of
    (as)       -> foo <$> as

with the following error:

 Cannot unify Control.Monad.Eff.Eff (reactive :: Control.Reactive.Reactive | u8437) u8438 with Prelude.Unit.

Why?

score 1 · Accepted Answer · edited Mar 24 '18 at 14:35

1

The error message is telling you that you're putting an Control.Monad.Eff.Eff (reactive :: Control.Reactive.Reactive | u8437) u8438 where a Unit should go.

Does foo have an type declaration? It probably won't work without one regardless of what you're doing in view, because of the current lack of typeclass constraint inference.

In the second case the type is:

foo :: forall a eff. Maybe a -> Eff (reactive :: Control.Reactive.Reactive | eff) a

The type of (<$>) is

(<$>) :: forall f a b. (Functor f) => (a -> b) -> f a -> f b

And the type of as will be Maybe (Linker Number a eff), so if you start substituting types you can see pretty quickly where it goes wrong:

a ~ Maybe (Linker Number a1 eff)
b ~ Eff (reactive :: Control.Reactive.Reactive | eff) (Linker Number a1 eff)
f ~ Maybe

So the f b result type of foo <$> as is:

Maybe (Eff (reactive :: Control.Reactive.Reactive | eff) (Linker Number a1 eff))

edited Mar 24 '18 at 14:35

Fyodor Soikin

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answered Aug 08 '14 at 15:19

gb.

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Ok its really Obvious now. Thank you. Sorry, I was just beating my head around my own ignorance. – Fresheyeball Aug 08 '14 at 15:24
Actually, I'm not sure that type substitution is entirely correct, but hopefully you can figure it out from there! – gb. Aug 08 '14 at 15:27
Well so far the type system has been the hardest part about transitioning to functional. I seem to spend 75% of my time just fighting with the type system, and in cases like this I was just lost. Sometimes I just try to write 2 logically equivalent things in different syntax's, when that does not work I'm just confused. – Fresheyeball Aug 08 '14 at 15:49

PureScript Type Error is hard to understand

1 Answers1