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This transcript tests an ability check failure regression.
structural ability Async t g where
fork : '{Async t g, g} a -> t a
await : t a -> a
Async.parMap : (a ->{Async t g, g} b) -> [a] ->{Async t g} [b]
Async.parMap f as =
tasks = List.map (a -> fork '(f a)) as
List.map await tasks
The issue was that certain ability processing was happing in less
optimal order. g
appears both as an ability used and as a parameter
to Async
. However, the latter occurrence is more strict. Unifying
the types Async t g1
and Async t g2
requires g1
and g2
to
be equal, while abilities that occur directly in a row are subject to
some subtyping.
However, the ability handling was just processing rows in whatever
order they occurred, and during inference it happened that g
occurred in the row before Async t g
. Processing the stricter parts
first is better, becauase it can solve things more precisely and avoid
ambiguities relating to subtyping.