Type family equation violates injectivity?

Dominick Samperi djsamperi at gmail.com
Sat Dec 29 18:49:41 UTC 2018

When I use v8.6.3 of GHC under Ubuntu to install the inline-r package
I get the error "Type family equation violates injectivity annotation," and
a type variable on the LHS cannot be inferred from the RHS, due to
the lack of injectivity (I suppose).

On the other hand, v8.0.2 of GHC (shipped with Haskell Platform under
Ubuntu) does not have this problem (it has other problems).

Has something changed in the latest version of the compiler that might
cause this? Possible work-around?

FYI, the line that triggers the error is:
type instance G.Mutable (W t ty s) = Mutable.W t ty

The variable that cannot be inferred is 's'.


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