Proposal for meta annotations in Elm

metaAnnotations.elm

-- REQUIRE META ANNOTATION:
-- The require meta-annotation will force the input record type vector to have a z field
require {z : Float} from vector
moveZ : Float -> vector -> vector
moveZ z v = { v | z <- v.z + z }


-- The top code is equivalent to
moveZ : Float -> { a | z : Float } -> { a | z : Float }
moveZ z v = { v | z <- v.z + z }

------------------

-- PROHIBIT META ANNOTATION
-- The prohibit meta-annotation will ban any input record type with an existing z field
prohibit {z : Float} from vector2d
add3dComponent : vector2d -> vector3d
add3dComponent v = { v | z = 0 }

-- This has no current equivalent in Elm

-------------------

-- OPTIONAL META ANNOTATION
-- The optional meta-annotation will declare that a field is optional and as such
-- must be properly handled. Checking is made with a built-in "has" function.
optional {z : Float} from vector
add : vector -> vector -> vector
add p q = 
  if (has {z : Float} p) 
  then
    {p | x <- p.x + q.x, y <- p.y + q.y, z <- p.z + q.z }
  else
    {p | x <- p.x + q.x, y <- p.y + q.y }
    

-- The above is not possible in Elm. The only way to achieve this 
-- is by splitting it into two distinct functions
add2 : {a | x : Float, y : Float} -> {b | x : Float, y : Float} -> {a | x : Float, y : Float }
add2 p q = {p | x <- p.x + q.x, y <- p.y + q.y }

add3 : {a | x : Float, y : Float, z : Float} ->
       {b | x : Float, y : Float, z : Float} ->
       {a | x : Float, y : Float, z : Float}
add3 p q = {p | x <- p.x + q.x, y <- p.y + q.y, z <- p.z + q.z}