Another vector library in Elm, which fails due to a suspected bug in the record type system?

console

[1 of 5] Compiling Vec2                ( Vec2.elm )
Type error on line 31:

        {x = v.x, y = v.y}

   Expected Type: a
     Actual Type: {}


Type error on line 34:

        origin

   Expected Type: {}
     Actual Type: a

make: *** [Main.html] Error 1

gistfile1.elm

module Vec2 where

{- 2D Vector -}

type Vec2Ext a = { a | x : Float, y : Float }

type Vec2 = Vec2Ext {}

origin : Vec2
origin = { x = 0, y = 0 }

scaleVec : Float -> Vec2Ext a -> Vec2Ext a
scaleVec a v = { v | x <- v.x * a, y <- v.y * a }

rotVec : Float -> Vec2Ext a -> Vec2Ext a
rotVec theta v =
  let newX = v.x * cos theta - v.y * sin theta
      newY = v.x * sin theta + v.y * cos theta
  in { v | x <- newX, y <- newY }

addVec : Vec2Ext a -> Vec2Ext b -> Vec2Ext b
addVec v1 v2 =
  let newX = v1.x + v2.x
      newY = v1.y + v2.y
  in { v2 | x <- newX, y <- newY }

flip : (a -> b -> c) -> (b -> a -> c)
flip f a b = f b a

extractVec : Vec2Ext a -> Vec2
extractVec v = { x = v.x, y = v.y }

sumVec : [ Vec2Ext a ] -> Vec2
sumVec = foldl (flip addVec) origin

The problem occurs with sumVec - and I think the issue is unifiying the extended type with the empty record type

flip should be built-in. Not sure though?

Actually, I think this should be a type error. I think the type of sumVec should be this:

sumVec : [ Vec2Ext a ] -> Vec2Ext a

I think the same would happen with a Haskell program that needs to unify free and bound type variables that are listed by the user.

I think this is the same as trying to write the following type:

specificListId :: [a] -> [()]
specificListId xs = xs

Which does not work in Haskell either.

I'll add a note about "cannot unify rigid type variable" or something. Probably something more comprehensible than that though :)

I pushed a change that provides better error messages.