Purescript: Minor documentation update to RowCons

[vladciobanu]

I would like to do a minor update to the RowCons documentation in Prim.hs:

RowCons l a i o can be read as o ~ { l :: a | i }

Perhaps also add a brief description saying what each of l, a, i and o represent. I know it took me a bit to figure out. I'll add a proposal for the changes tonight.

[hdgarrood]

Additionally, we could probably use words for some of those type variables while we're at it:

class RowCons (label :: Symbol) (a :: Type) (in_row :: # Type) (out_row :: # Type) | [...]

Also, I've just realised this should probably wait until #3176 is merged, in order to avoid causing merge conflicts.

[joneshf]

As mentioned i n the chat, it should be o ~ ( l :: a | i ), since it works for any row of types, not just records.

[LiamGoodacre]

As RowCons also works with l o -> a i, I'd suggest tail and row instead of in_row and out_row. I feel this would be less confusing - than if you are using it this way but still have to read it as in and out.

[vladciobanu]

Environment.hs changes from
class RowCons (l :: Symbol) (a :: Type) (i :: # Type) (o :: # Type) | l a i -> o, l o -> a i
to
class RowCons (label :: Symbol) (a :: Type) (tail :: # Type) (row :: # Type) | label a tail -> row, label row -> a tail

Prim.hs changes from

The RowCons type class is a 4-way relation which asserts that one row of types can be obtained from another by inserting a new label/type pair on the left.

to

The RowCons type class is a 4-way relation which asserts that one row of types can be obtained from a "tail" row by inserting a new label/type pair to the tail (row ~ (label :: a | tail)).

Not super happy with _a "tail" row_ bit, but I'm not sure how to phrase it more elegantly. I'll wait with the PR until #3176 is merged.