CODEWITHSUNDEEP
purescript
topica
topica

Reputation: 319

purescript nested row polymorphism

PureScript by Example 5.9 Exercise 1 and 2

My Solution:

type HasCity r s = { address :: { city :: String | r } | s }

livesInLA :: forall r s. HasCity r s -> Boolean
livesInLA { address: { city: "Los Angeles" } } = true
livesInLA _ = false

sameCity :: forall r s t u. HasCity r s -> HasCity t u -> Boolean
sameCity a b = a.address.city == b.address.city

Question:

forall r s t u. HasCity r s -> HasCity t u is nasty... Can it be simplified?

Upvotes: 3

Views: 170

Answers (1)

gb.
gb.

Reputation: 4659

If you want to preserve the rows as having optional fields, not really. The only slight simplification I can suggest would be:

forall r s. HasCity r s -> HasCity r s -> Boolean

But this would require that both arguments have exactly the same structure (while still allowing extra labels), whereas the original type only requires that each record has the fields used in the equality test.

Upvotes: 1

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