Data.Functor.void needs a way to specify the type of discarded value

[yaitskov]

I have following code snippet:

post :: forall m a. DecodeJson a => Monad m => String -> m a 

deleteFoo = do
  void $ post "http://localhost/delete"

type checker complains with:

No type class instance was found for
    Data.Argonaut.Encode.Class.DecodeJson t3

Current version of void function cannot get type parameter:

deleteFoo = do
  void @Unit $ post "http://localhost/delete"

An expression of polymorphic type
  with the invisible type variable f:

    forall f a. Functor f => f a -> f Unit

  cannot be applied to:

    Boolean

I suggest to rewrite void function to make it more flexible:

voidAt :: forall @a f . Functor f => f a -> f Unit
voidAt = void

[JordanMartinez]

I suggest to rewrite void function to make it more flexible:

Is post code you control? Or from something else? post could also use a VTA arg to make this
void $ post @Unit "url"

And void would likely be updated to void @f first and possibly void @f @a since usually the f is more often the thing to specify than that a.

[garyb]

I would agree that this seems more like something that should be solved at post - I think when designing APIs I'd only be using VTAs to dictate values that are produced - it would never even occur to me to use them to dictate the input to a function, since I think of inference working that way around.