Search code examples
purescript

Combine Records in Purescript

Given I have the folowing records in purescript:

let name = {name: "Jim"}
let age = {age: 37}

is it possible to combine those two records some how in a generic way? Something like:

name 'comb' age

such that I get the following record:

{name: "Jim", age: 37}

Somehow it seems to be possible with the Eff rowtype, but I'm curious if it would be possible with 'normal' records. I'm new to purescript and it's record syntax.

Thanks a lot.

Solution

  • note: When this answer was accepted, it was true, but now we do have the row constraints it mentions, and a library for manipulating records that includes merges/unions: https://github.com/purescript/purescript-record

    It's not possible to do this at the moment, as we don't have a way of saying that a row lacks some label or other. It is possible to have an open record type:

    something :: forall r. { name :: String | r } -> ...

    But this only allows us to accept a record with name and any other labels, it doesn't help us out if we want to combine, extend, or subtract from records as it stands.

    The issue with combining arbitrary records is we'd have a type signature like this:

    comb :: forall r1 r2. { | r1 } -> { | r2 } -> ???

    We need some way to say the result (???) is the union of r1 and r2, but also we'd perhaps want to say that r1's labels do not overlap with r2's.

    In the future this may be possible via row constraints.