[Haskell-cafe] Safe Haskell?

[Richard Eisenberg]

> On May 3, 2021, at 7:40 AM, Tom Smeding <x at tomsmeding.com> wrote:
> 
> But perhaps I'm being ignorant of other existing cases where this
> already matters, and I've been living in an idealised world until now.

Sad to say it, but order does matter here.

In the very simple case, if you have {-# LANGUAGE FlexibleContexts, NoFlexibleContexts #-}, that's different from {-# LANGUAGE NoFlexibleContexts, FlexibleContexts #-} -- later extensions override earlier ones. This problem becomes more confounding when we recognize that some extensions imply others. For example {-# LANGUAGE TypeFamilies, NoMonoLocalBinds #-} means something different from {-# LANGUAGE NoMonoLocalBinds, TypeFamilies #-} because TypeFamilies implies MonoLocalBinds. Perhaps even worse, {-# LANGUAGE CUSKs, StandaloneKindSignatures #-} differs from {-# LANGUAGE StandaloneKindSignatures, CUSKs #-} because StandaloneKindSignatures implies NoCUSKs.

Returning to Safe Haskell:

It's true that Safe cannot be overridden locally. This is implemented by the fact that NoSafe does not exist. To me, this design makes sense, because it means that compiling with `ghc -XSafe` is guaranteed to use Safe Haskell. So we would need something like a default-safety field in Cabal, that could be overridden locally.

But, still, this may be easier than the status quo.

Do we think this would work? Specifically:

* Introduce a new flag -fdefault-safety={safe,trustworthy,unsafe} that changes the module-level default. This default names the safety level in effect for any module that declares none of Safe, Trustworthy, or Unsafe.
* If -fdefault-safety is not specified at the command line, it is as if the user wrote -fdefault-safety=unsafe.

And that's it.

Consequence: Safe-inference would never take place, because every module would have a declared level of Safety. The Safe-inference code could thus be removed.

Further work: Introduce default-safety in Cabal, but that's not really necessary to make the changes above.

What do we think?

Richard
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