Pattern matching on two records with the same fields

Question

Say I have this record:

type alias Rec = { a : Int }

And, for example, a function that takes two of these and sums their integers.

f: Rec -> Rec -> Int

This can be implemented using record accessors (i.e. f x y = x.a + y.a), but is there a way to use pattern matching to extract both integers?

Obviously, these two do not work because they would be binding two different numbers to the same variable:

f {a} {a} = a + a

f x y = case (x, y) of ({a}, {a}) -> a + a

Answer 1

There seems to be no such way in the current Elm language. In other functional languages such as ML and Haskell, you could write patterns inside records like:

$ sml
Standard ML of New Jersey v110.74 [built: Sat Oct  6 00:59:36 2012]
- fun func {field=x} {field=y} = x+y ;
val func = fn : {field:int} -> {field:int} -> int
- func {field=123} {field=45} ;
val it = 168 : int

You might as well make a feature request to the developer(s) of Elm - or ask a question in the community mailing list at least.

P.S. After a quick search, I found such a proposal to add ML-like pattern matching on record fields in Elm, but it seems to have been turned down.:-(

Answer 2

There's no way to do this currently. There is pattern aliasing (as) but it only works for a whole pattern, so this is invalid:

type alias Rec = { a : Int }

f: Rec -> Rec -> Int
f { a as xa } { a as ya } = xa + ya

main = f { a = 1 } { a = 2 }

results in:

Detected errors in 1 module.


-- SYNTAX PROBLEM --------------------------------------------------------------

I ran into something unexpected when parsing your code!

4| f { a as xa } { a as ya } = xa + ya
         ^
I am looking for one of the following things:

    a closing bracket '}'
    whitespace