[Haskell-cafe] Safe Haskell?

[Carter Schonwald]

Default safety isn’t quite what you want in normal packages, because that
disables all user land rewrite rules! It may have other implications too,
but short of augmenting ghc with a proof system for correctness of rewrite
rules, default safehaskell is at odds with optimized builds.

On Mon, May 3, 2021 at 10:28 AM Richard Eisenberg <rae at richarde.dev> wrote:

>
>
> 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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