How do Coercions work

Jan van Brügge jan at
Mon Jul 22 10:14:11 UTC 2019


currently I have some problems understanding how the coercions are
threaded through the compiler. In particular the function
`normalise_type`. With guessing and looking at the other cases, I came
to this solution, but I have no idea if I am on the right track:

normalise_type ty
  = go ty
    go (RowTy k v flds)
      = do { (co_k, nty_k) <- go k
           ; (co_v, nty_v) <- go v
           ; let (a, b) = unzip flds
           ; (co_a, ntys_a) <- foldM foldCo (co_k, []) a
           ; (co_b, ntys_b) <- foldM foldCo (co_v, []) b
           ; return (co_a `mkTransCo` co_b, mkRowTy nty_k nty_v $ zip
ntys_a ntys_b) }
            foldCo (co, tys) t = go t >>= \(c, nt) -> return (co
`mkTransCo` c, nt:tys)

RowTy has type Type -> Type -> [(Type, Type)]

What I am not sure at all is how to treat the coecions. From looking at
the go_app_tys code I guessed that I can just combine them like that,
but what does that mean from a semantic standpoint? The core spec was
not that helpful either in that regard.

Thanks in advance


More information about the ghc-devs mailing list