[Haskell-cafe] Defining a Strict language pragma

Stephen Chang stchang at ccs.neu.edu
Tue Nov 6 20:25:19 CET 2012

Hi Johan (and haskell-cafe),

I work on the Racket language (but I'm also interested in Haskell so
I've been following this list and I am consistently impressed with
what the Haskell community has produced!). In Racket, we are very
interested in the problem of inter-language operation, like what you
suggested, with the goal of creating a framework that supports
programs where each module potentially uses a different language.

You might be interested in checking out Matthews and Findler's work on
interlanguage interoperability, which addresses the theory of how
languages can interoperate in a safe manner.

On the practical side, we have had success getting Typed Racket and
(untyped) Racket to safely interoperate by using contracts at module
boundaries to specify and enforce the desired program properties. See
Sam Tobin Hochstadt's dissertation for more details.

For lazy-strict language interoperation, I'm not sure if a pragma is
sufficient. Matthias Felleisen and I have looked into this but we've
run into problems since in
a higher-order language, strict and lazy functions can easily flow back
and forth between the two worlds and like you mentioned, it's not
clear how we want each to behave. For example, in a strict module, if
you define a function, you can avoid thunking the arguments when the
function is called, but what happens if you export the function and
use it in a lazy module, which passes in a thunk. Do you have to
preemptively insert bangs in your strict function to handle this? We
have considered something similar to the Typed Racket solution --
adding strictness
patterns at boundaries to describe the interactions -- analogous
to higher-order contracts -- but it is not clear if this is sufficient
to make all flows safe.

On Mon, Nov 5, 2012 at 5:52 PM, Johan Tibell <johan.tibell at gmail.com> wrote:
> Hi all,
> I would like to experiment with writing some modules (e.g. low-level modules
> that do a lot of bit twiddling) in a strict subset of Haskell. The idea is
> to remove boilerplate bangs (!) and instead declare the whole module strict.
> I believe this would both make code that needs to be strict more declarative
> (you say what you want once, instead of putting bangs everywhere), less
> noisy, and more predictable (no need to reason about laziness in places
> where you know you don't want it). The idea is to introduce a new language
> pragma
> {-# LANGUAGE Strict #-}
> that has the above described effect.
> The tricky part is to define the semantics of this pragma in terms of
> Haskell, instead of in terms of Core. While we also need the latter, we
> cannot describe the feature to users in terms of Core. The hard part is to
> precisely define the semantics, especially in the presence of separate
> compilation (i.e. we might import lazy functions).
> I'd like to get the Haskell communities input on this. Here's a strawman:
>  * Every function application f _|_ = _|_, if f is defined in this module
> [1]. This also applies to data type constructors (i.e. the code acts if all
> fields are preceded by a bang).
>  * lets and where clauses act like (strict) case statements.
>  * It's still possible to define strict arguments, using "~". In essence the
> Haskell lazy-by-default with opt-out via "!" is replaced with
> strict-by-default with opt-out via "~".
> Thoughts?
> Cheers,
>   Johan
> 1. I could see the very opposite choice be made: function applications to
> functions defined in other modules are also strict. We should try to think
> about the implications of either policy.
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