[GHC] #11824: GHC error in desugarer lookup

[Ben Gamari]

Simon Peyton Jones <simonpj at microsoft.com> writes:

> Ben
>
> I'm offline, so can't reply on Phab.
>
> Calling mkTypeableBinds in tcRnHsBootDecls isn't quite right, because it'll generate lots of *bindings* whereas all we want is the *Ids* in the GlobalValEnv.
>
> (It's possible that tcg_binds is ignored in the boot-module case, but it's unsavoury to generate the bindings at all.  They are generated when we process the main module.)
>
> I suggest the easiest fix is, in mkModIdBinding and mkTypeableTyConBinds, guard the binding-generation with a test for whether isHsBootOrSig is true.
>
Are you concerned about the bindings being generated at all or merely
that they are being added to the typechecking environment?

If the latter there is no reason to fear: TcBinds.addTypecheckedBinds
(which TcTypeable uses) ensures that no bindings are added to the
environment if we are compiling a boot module,

    addTypecheckedBinds :: TcGblEnv -> [LHsBinds Id] -> TcGblEnv
    addTypecheckedBinds tcg_env binds
      | isHsBootOrSig (tcg_src tcg_env) = tcg_env
        -- Do not add the code for record-selector bindings
        -- when compiling hs-boot files
      | otherwise = tcg_env { tcg_binds = foldr unionBags
                                                (tcg_binds tcg_env)
                                                binds }

Of course, we are still doing the work of producing the bindings; if it
is this that you are concerned about then I can be more careful in
guarding their production.

Cheers,

- Ben
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