Change the meaning of -fextended-default-rules

[simonmar]

Currently -fextended-default-rules applies to source code modules. GHCi has the extended defaulting behaviour by default for expressions typed at the prompt, and there's no way to change that.

We propose the following change: extended defaulting would only ever apply to expressions typed at the GHCi prompt, and it would be disabled with -fno-extended-default-rules (on by default).

See also #1200, where -fextended-default-rules was introduced.

[sorear]

Note that this will break lambdabot, which wraps user-provided code in a module and compiles it with -fextended-default-rules and -O2. Some other method will need to be found to access that combination of functionality.

[igloo]

OK, thanks for pointing that out, sorear. It sounds like we need 2 flags then: One to control behaviour for code typed at the GHCi prompt, and one for the behaviour for code from files. In which case we should leave the existing flag with its current meaning and add one to control the behaviour for code typed at the GHCi prompt.

[SamB]

Replying to igloo:

OK, thanks for pointing that out, sorear. It sounds like we need 2 flags then: One to control behaviour for code typed at the GHCi prompt, and one for the behaviour for code from files. In which case we should leave the existing flag with its current meaning and add one to control the behaviour for code typed at the GHCi prompt.

What about the -X flag? We could use that one for source files and -fextended-default-rules for GHCi.

[simonpj]

I'm missing something.

• If you put -fextended-default-rules (or a corresponding -X language flag) in a module, it should apply to that module.
• If you :set -fno-extended-default-rules at the GHCi prompt, it should switch it off for GHCi.

Ah, hmm, maybe I see. Say you :set -fallow-overlapping-instances at the GHCi prompt. That setting also applies when GHCi compiles a module, doesn't it? If you didn't want overlapping instances in the module there's no way to switch it off, short of putting -fno-allow-overlapping-instances in the module, which seems wrong.

But in the case of extended-defaults, GHCi typically has it on, but we don't want to have it on by default for a module. That's a new situation. An ad-hoc solution would be to splat the flags with -fno-extended-defaults just before GHCi compiles a module.