rescript-lang/rescript-lang.org
Document co / contravariance (`<+a>`, `<-a>`)
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#149 opened on 2021/01/01
help wantedmanual
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説明
This one is definitely an advanced topic.
- What does
+and-mean? - What syntax constructs are possible? (type, function, etc.)
- What practical use-cases does it have / Why do we need it?
Things to avoid:
- Complex / too scientific wording
- Too generic examples without practical context
Hints:
- We use a covariant type in
Js.Promise.t(see it here)