Finding & assembling class dictionaries from GHC plugins?

[Conal Elliott]

Hi Simon,

Here's some context on what I'm after. I'll try to include enough to
clarify what I'm after, without dragging you into unnecessary detail.

I have a Core expression, which I'm systematically translating to a related
Core expression. The second one reifies the first one, so that the
generated code *evaluates* to a run-time (not compile/Core-time)
representation akin to the original Core expression. In this second
(reified) representation, I need some run-time representation of types. I
use Typeable constraints in my representation, and these constraints don't
appear in the original Core representation. For instance, I have the
following constructor for function application in a GADT of lambda
expressions:

> (:^) :: (Typeable a, Typeable b) => E (a -> b) -> E a -> E b

My reification plugin takes a Core application expression, recursively
reifies the function and argument expressions, and makes an expression that
applies (:^) to the two recursive results. However--and the point of this
thread--I also need to come up with Core expressions for the two Typeable
dictionaries.

So, to clarify my request, given a Core type representation t, I want to
construct a Core expression that evaluates to the dictionary for Typeable t
if one exists, or fail if there isn't one. Note that t is not necessarily
an atomic type; it could instead be something like [(Bool,Int -> String)].

I'm happy to Skype-chat if it'd help.

Thanks,  - Conal



On Tue, Nov 5, 2013 at 2:37 PM, Simon Peyton-Jones <simonpj at microsoft.com>wrote:

>  We just don't know how to start — ie how call the right functions from
> the type checker. Are there any resources explaining the relevant subset of
> the TcRnIf API?
>
> And I for my part don’t know where to start helping you!  Conal wrote
>
>
>
> I don't think I need to construct dictionaries for new Typeable instances.
> I only need to find and assemble *existing* Typeable instances into
> combinations like Typeable [(Bool,Int -> String)], and I don't know how to
> do so in a GHC plug-in
>
>
>
> That’s exactly what the constraint solver does.  (TcSimplify and
> friends.)  But again I need more context.
>
>
>
> I gather you are talking to Pedro too?  He knows a lot about this stuff.
>
>
>
> Also there are a bunch of folk (Luite, Edsko) worked on the new front-end
> plugin stuff, and are much more expert in it than me.
>
>
>
> We could have a skype call if that would help
>
>
>
> S
>
>
>
>
>
> *From:* Nicolas Frisby [mailto:nicolas.frisby at gmail.com]
> *Sent:* 05 November 2013 14:36
> *To:* Simon Peyton-Jones
> *Cc:* ghc-devs at haskell.org; Conal Elliott
> *Subject:* RE: Finding & assembling class dictionaries from GHC plugins?
>
>
>
> On Nov 5, 2013 3:14 AM, "Simon Peyton-Jones" <simonpj at microsoft.com>
> wrote:
>
> >
> > A core-level plug-in can’t generate fresh instances.  There are some new
> plug-in hooks that fit earlier in the pipeline, which can.  Maybe you can
> use that, and generate a data type decl with “deriving Typeable”?
>
> We're willing to do some awkward shoehorning in HERMIT to make this work
> in limited circumstances. (Hopefully including Conal's.)
>
> We just don't know how to start — ie how call the right functions from the
> type checker. Are there any resources explaining the relevant subset of the
> TcRnIf API?
>
> Thanks.
>
> >
> >
> > From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of Conal
> Elliott
> > Sent: 04 November 2013 20:08
> > To: ghc-devs at haskell.org
> > Subject: Finding & assembling class dictionaries from GHC plugins?
> >
> >
> >
> > I'm working on a GHC plugin (for compiling Haskell to hardware), and I
> need to synthesize Typeable (and maybe other) class dictionaries for a wide
> range of types, including composite types (functions, pairs, lists, etc).
> Can it be done, and how? I think I'm mainly looking for mechanics of
> finding existing class instances (however they're defined) and assembling
> them (for parametrized/composite dictionaries), rather than mechanisms
> specific to Typeable.
> >
> > Thanks, -- Conal
> >
> >
> > _______________________________________________
> > ghc-devs mailing list
> > ghc-devs at haskell.org
> > http://www.haskell.org/mailman/listinfo/ghc-devs
> >
>
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