Assigning record to a record that has a subset of fields

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#1

What’s the reason records cannot be assigned to a record that has a subset fields?

type A = {a :: Int}
type B = {a :: Int, b :: Int}

bx :: B
bx = {a: 1, b: 2}

ax :: A
ax = (\{a} -> {a}) bx -- Works
ax = bx -- Compile error: Type of expression contains additional label b.
``
#2

Hi,

the types are different:

  • bx is B
  • you claim a to be A but assign an B - there is no automatic casting/coercing and no subtype casting in PureScript

the middle one works as:

\{a} -> \{a}

has basically

\forall t r. { a :: t | r } -> { a :: t }
#3

Is there a way to do it manually without introducing the additional \{a} -> {a} function?

#4

Well you can use the forall ... r. ... { a :: t | r } -> ... type everywhere you want to use it like that. Or give the function some name and use it.

There is probably something like this already defined in some library and unsafeCoerce should work too.

1 Like
#5

record-studio has a shrink function that you can use:

import Prelude

import Effect (Effect)
import Effect.Console (log)
import Record.Studio (shrink)

type A = { a :: Int }
type B = { a :: Int, b :: Int }

bx :: B
bx = { a: 1, b: 2 }

ax :: A
ax = shrink bx
3 Likes