How to have row without particular field as function param

Question

What I want is following, I have a row type:

type FooBarBaz =
  ( foo :: Int
  , bar :: String
  , baz :: Boolean
  )

Then a need a function that takes this row type record but without "foo": and adds default "foo" field:

withoutFoo { bar: "bar", baz: true} -- > { foo: 1, bar: "bar", baz: true}

I tried to do something like this, but can not get it around:

withoutFoo :: ∀ r.
  Lacks "foo" r =>
  Cons "foo" Int r FooBarBaz =>
  { | r } -> { | FooBarBaz }
withoutFoo props = Record.union { foo: 1 } props

UPD:

managed to solve it with Union constraint:

withoutFoo :: ∀ r.
  Union r ( foo :: Int ) FooBarBaz =>
  { | r } -> { | FooBarBaz }
withoutFoo props =
  Record.merge props { foo: 1 }

Please don't tag your titles. Integrate them into your title question if appropriate. — isherwood, Jun 13 '22 at 19:10

score 1 · Accepted Answer · answered Jun 13 '22 at 19:23

1

The problem is that your function isn't actually generic. There is only one possible choice for r: it should always be (bar :: String, baz :: Boolean)

So just make that the type of your function:

withoutFoo :: { bar :: String, baz :: Boolean } -> { | FooBarBaz }
withoutFoo props = Record.union { foo: 1 } props

answered Jun 13 '22 at 19:23

Fyodor Soikin

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Thanks, though actually managed to solve it with Union constraint and Record.merge (updated OP). – WHITECOLOR Jun 13 '22 at 19:34
That's a needless complication. In your solution, `r` is always `(bar :: String, baz :: Boolean)` – Fyodor Soikin Jun 13 '22 at 21:50
In the real use case I have quite a big row, that consists of many more than tree fields, and I resuse it in other places as well -) – WHITECOLOR Jun 14 '22 at 09:59

How to have row without particular field as function param

1 Answers1