generalize variables bound by <-
#1501
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xsebek
approved these changes
Sep 9, 2023
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As a note for the future, we may possibly want to reconsider this at some point; see "Let Should Not Be Generalized" by Vytiniotis, Peyton Jones, and Schrijvers ( https://www.microsoft.com/en-us/research/wp-content/uploads/2016/02/tldi10-vytiniotis.pdf ). But I think this will be fine for now. |
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Closes #351.
I sat down with @ccasin today to look at #351. He convinced me that it is actually fine to return
forall a. a -> aas the type ofrinr <- return (\i.i), even thoughreturn (\i.i)has typeforall a. cmd (a -> a)and we can never have a typecmd (forall a. a -> a). Basically, in a Hindley-Milner-style system it is always safe to generalize at any time.The real problem --- which I was very close to discovering in #351 (comment) --- is simply that we need to generalize the type of variables bound by
<-. However, we need to make sure to do so before putting that variable in the context, which I apparently failed to do when I tried fixing this before.