PolyMorphicAttrs.purs

module Main where

import Prelude (Unit)
import Control.Monad.Eff (Eff)
import Control.Monad.Eff.Console (CONSOLE, log)
import Data.Function.Eff (EffFn4, EffFn3, mkEffFn4, runEffFn3)

foreign import data D3 :: !
foreign import data Selection :: * -> *
foreign import data D3Element :: *

type PredicateFn    d x = ∀ eff. (d -> Number -> (Array D3Element) -> D3Element -> Eff (d3::D3|eff) x)

data AttrSetter d x = SetAttr x
                    | AttrFn (PredicateFn d x)   -- rename both data ctor and Type here TODO


foreign import attrFn  :: ∀ d v eff. EffFn3 (d3::D3|eff) String v                    (Selection d) (Selection d)
foreign import attrFnP :: ∀ d x eff. EffFn3 (d3::D3|eff)         -- eff of attrFnP
                                             String              -- 1st arg of attrFnP
                                             (EffFn4 (d3::D3|eff)       -- eff of callback (callback is 2nd arg of fn)
                                                      d                 -- 1st arg of callback
                                                      Number            -- 2nd arg of callback
                                                      (Array D3Element) -- 3rd arg of callback
                                                      D3Element         -- 4th arg of callback
                                                      x)                -- result of callback, what the attr is to be set to
                                             (Selection d)       -- 3rd arg of attrFnP
                                             (Selection d)       -- result of attrFnP

attr :: ∀ d x eff. String -> AttrSetter d x      -> Selection d -> Eff (d3::D3|eff) (Selection d)
attr s (SetAttr x)           = runEffFn3 attrFn  s x
attr s (AttrFn p)            = runEffFn3 attrFnP s (mkEffFn4 p)

main :: forall e. Eff (console :: CONSOLE | e) Unit
main = do
  log "Hello sailor!"

Very weird, code above compiles but in more meaty example line 32 (mkEffFn4 p) fails as follows:

117  attr s (AttrFn p)            = runEffFn3 attrFnP s (mkEffFn4 p)
                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  Could not match type

    d0

  with type

    x3

  while trying to match type Selection d0
    with type Selection t2
  while checking that expression ((runEffFn3 attrFnP) s) (mkEffFn4 p)
    has type Selection d0
             -> Eff
                  ( d3 :: D3
                  | eff1
                  )
                  (Selection d0)
  in value declaration attr

  where eff1 is a rigid type variable
          bound at line 116, column 1 - line 116, column 53
        x3 is a rigid type variable
          bound at line 116, column 1 - line 116, column 53
        d0 is a rigid type variable
          bound at line 116, column 1 - line 116, column 53
        t2 is an unknown type

Link to source line of that error in the purescript-d3v4 repo:
https://github.com/afcondon/purescript-d3v4/blob/newstart/src/Selection.purs#L117