On Wed, Jun 10, 2015 at 3:21 PM, Nathan Bouscal
The current behavior may be surprising if you are not aware of it, but it's the only sensible one. Otherwise, what should the meaning of
{-# LANGUAGE MonoLocalBinds, NoMonoLocalBinds #-}
be?
Arguably it should be a compiler warning.
+1. I'd say, when a set of flags are mutually incompatible (e.g., {Foo, NoFoo}) then that should be a warning/error. Whereas with GADTs, we *recommend* (and by default assume) MonoLocalBinds, but GADTs are not *incompatible* with NoMonoLocalBinds. We already support defeasible recommendations (in practice, if not intentionally); why not remove the counterintuitive corner cases they create in the current implementation? As for the complaint about project-wide vs file-specific language extensions: yes, it can be helpful to have overriding behavior; but having that behavior at the commandline or within a single file is counterintuitive at best. What I think would make more sense is a set-valued variant of the Last monad where each local collection of constraints (the collection given on the commandline, the collection given in the cabal file, the collection given in the Haskell file itself, etc) must be internally consistent, generating an error/warning if it is not. And then we simply have the more fine-grained constraint sets override the coarser-grained constraint sets. (The only conceptual infelicity I can see to that approach is when we enable certain flags because of other ones, like LiberalEverything whenever we have FunDeps. In principle, if a constraint set further down the line says NoFunDeps, then this should also disable the LiberalEverything that depended on FunDeps —or at least, someone may desire that behavior. Resolving this would require a rather sophisticated model of intent behind enabling various language extensions. A model I do not propose developing at present. However, since we currently lack such facilities, we would lose nothing by not having them in the simple approach proposed above.) -- Live well, ~wren