Thanks for the helpful reply, Simon!

> > The new bit here is that `$dC'` is not found via matching in the LHS, but

> > rather by instance resolution from `k`, which does appear explicitly in

> > the LHS

 

> Well this would be something qualitatively new. We don’t that ability in

> rules; and it’s far from clear to me what it would mean anyway. I suppose

> that if k was instantiated to a ground type then you might hope to solve it,

> but what if it was instantiated to some in-scope type variable t, and some

> variable of type (C t) was in scope. Should that work too?

Yes, we'd want to use in-scope dictionary variables to help solve the needed

constraints in the presence of polymorphism. I've run into exactly this need

in my own manually programmed ("built-in") rewrite rules. Would it be possible

to do so (without deep/pervasive changes to GHC)?

> Happily it sounds as if you are making progress with help from Joachim.

No, I think Joachim agrees that it's impractical to write all of the needed

rule specializations, and that generating then programmatically would be less

convenient than the implementing the built-in rules as I do now.

Cheers, -- Conal

On Wed, Oct 4, 2017 at 2:08 AM, Simon Peyton Jones <simonpj@microsoft.com> wrote:

The new bit here is that `$dC'` is not found via matching in the LHS, but rather by instance resolution from `k`, which does appear explicitly in the LHS

 

Well this would be something qualitatively new.  We don’t that ability in rules; and it’s far from clear to me what it would mean anyway.  I suppose that if k was instantiated to a ground type then you might hope to solve it, but what if it was instantiated to some in-scope type variable t, and some variable of type (C t) was in scope.  Should that work too?

 

I’m highly dubious.

 

Happily it sounds as if you are making progress with help from Joachim.

 

Simon

 

From: conal.elliott@gmail.com [mailto:conal.elliott@gmail.com] On Behalf Of Conal Elliott
Sent: 03 October 2017 16:30
To: Simon Peyton Jones <simonpj@microsoft.com>
Subject: Re: GHC rewrite rule type-checking failure

 

 

The revised example I gave earlier in the thread:

 

``` haskell

class C k where comp' :: k b c -> k a b -> k a c

 

instance C (->) where comp' = (.)

 

-- Late-inlining version to enable rewriting.

comp :: C k => k b c -> k a b -> k a c

comp = comp'

{-# INLINE [0] comp #-}

 

morph :: (a -> b) -> k a b

morph = error "morph: undefined"

 

{-# RULES "morph/(.)" forall f g. morph (g `comp` f) = morph g `comp` morph f #-}

 

-- • Could not deduce (C k) arising from a use of ‘comp’

--   from the context: C (->)

--     bound by the RULE "morph/(.)"

```

 

A hypothetical generated Core rule (tweaked slightly from Joachim's note):

 

``` haskell

forall (@ k) (@ b) (@ c) (@ a)

       ($dC :: C (->))

       (f :: a -> b) (g :: b -> c).

  morph @ k @ a @ c (comp @ (->) @ b @ c @ a $dC g f)

  = comp @ k @ b @ c @ a

      $dC'

      (morph @ k @ b @ c g)

      (morph @ k @ a @ b f)

 where

   $dC' :: C k

```

 

The new bit here is that `$dC'` is not found via matching in the LHS, but rather by instance resolution from `k`, which does appear explicitly in the LHS. If `C k` cannot be solved, the rule fails. The "where" clause here lists the dictionary variables to be instantiated by instance resolution rather than matching.

 

-- Conal

 

 

On Tue, Oct 3, 2017 at 8:01 AM, Simon Peyton Jones <simonpj@microsoft.com> wrote:

But synthesising from what?

 

And currently no: there is no type-class dictionary synthesis after the type checker.  Not impossible I suppose, but one more fragility: a rule does not fire because some synthesis thing didn’t happen.    Maybe give an as-simple-as-poss example of what you have in mind, now you understand the situation better?   With all the type and dictionary abstractions written explicitly…

 

S

 

From: conal.elliott@gmail.com [mailto:conal.elliott@gmail.com] On Behalf Of Conal Elliott
Sent: 03 October 2017 15:56
To: Simon Peyton Jones <simonpj@microsoft.com>

Subject: Re: GHC rewrite rule type-checking failure

 

Thanks, Simon. Your explanation make sense to me. Do you think that the rewrite rule mechanism could be enhanced to try synthesizing the needed dictionaries after LHS matching and before RHS instantiation? I'm doing as much now in my compiling-to-categories plugin, but without the convenience of using concrete syntax for the rules.

 

Regard, - Conal

 

 

On Tue, Oct 3, 2017 at 12:27 AM, Simon Peyton Jones <simonpj@microsoft.com> wrote:

*   Is it feasible for GHC to combine the constraints needed LHS and RHS to form an applicability condition?

I don’t think so.

 

Remember that a rewrite rule literally rewrites LHS to RHS.  It does not conjure up any new dictionaries out of thin air.  In your example, (D k b) is needed in the result of the rewrite.  Where can it come from?  Only from something matched on the left.

 

So GHC treats any dictionaries matched on the left as “givens” and tries to solve the ones matched on the left.  If it fails you get the sort of error you see.

 

One way to see this is to write out the rewrite rule you want, complete with all its dictionary arguments. Can you do that?

 

Simon

 

From: Glasgow-haskell-users [mailto:glasgow-haskell-users-bounces@haskell.org] On Behalf Of Conal Elliott
Sent: 03 October 2017 01:03
To: Joachim Breitner <mail@joachim-breitner.de>
Cc: glasgow-haskell-users@haskell.org
Subject: Re: GHC rewrite rule type-checking failure

 

Thanks very much for the reply, Joachim.

 

Oops! I flubbed the example. I really `morph` to distribute over an application of `comp`. New code below (and attached). You're right that I wouldn't want to restrict the type of `morph`, since each `morph` *rule* imposes its own restrictions.

 

My questions:

 

*   Is it feasible for GHC to combine the constraints needed LHS and RHS to form an applicability condition?

*   Is there any way I can make the needed constraints explicit in my rewrite rules?

*   Are there any other work-arounds that would enable writing such RHS-constrained rules?

 

Regards, -- Conal

 

``` haskell

{-# OPTIONS_GHC -Wall #-}

-- Demonstrate a type checking failure with rewrite rules

 

module RuleFail where

 

class C k where comp' :: k b c -> k a b -> k a c

 

instance C (->) where comp' = (.)

 

-- Late-inlining version to enable rewriting.

comp :: C k => k b c -> k a b -> k a c

comp = comp'

{-# INLINE [0] comp #-}

 

morph :: (a -> b) -> k a b

morph = error "morph: undefined"

 

{-# RULES "morph/(.)" forall f g. morph (g `comp` f) = morph g `comp` morph f #-}

 

-- • Could not deduce (C k) arising from a use of ‘comp’

--   from the context: C (->)

--     bound by the RULE "morph/(.)"

```

 

 

On Mon, Oct 2, 2017 at 3:52 PM, Joachim Breitner <mail@joachim-breitner.de> wrote:

Hi Conal,

The difference is that the LHS of the first rule is mentions the `C k`
constraint (probably unintentionally):

*RuleFail> :t morph comp
morph comp :: C k => k1 (k b c) (k a b -> k a c)

but the LHS of the second rule side does not:

*RuleFail> :t morph addC
morph addC :: Num b => k (b, b) b



A work-around is to add the constraint to `morph`:

    morph :: D k b => (a -> b) -> k a b
    morph = error "morph: undefined"

    but I fear that this work-around is not acceptable to you.

    Joachim

    Am Montag, den 02.10.2017, 14:25 -0700 schrieb Conal Elliott:
    > -- Demonstrate a type checking failure with rewrite rules
>
> module RuleFail where
>
> class C k where comp' :: k b c -> k a b -> k a c
>
> instance C (->) where comp' = (.)
>
> -- Late-inlining version to enable rewriting.
> comp :: C k => k b c -> k a b -> k a c
> comp = comp'
> {-# INLINE [0] comp #-}
>
> morph :: (a -> b) -> k a b
> morph = error "morph: undefined"
>
> {-# RULES "morph/(.)" morph comp = comp #-}  -- Fine



> class D k a where addC' :: k (a,a) a
>
> instance Num a => D (->) a where addC' = uncurry (+)
>
> -- Late-inlining version to enable rewriting.
> addC :: D k a => k (a,a) a
> addC = addC'
> {-# INLINE [0] addC #-}
>
> {-# RULES "morph/addC" morph addC = addC #-}  -- Fail
>
> -- • Could not deduce (D k b) arising from a use of ‘addC’
> --   from the context: D (->) b
>
> -- Why does GHC infer the (C k) constraint for the first rule but not (D k b)
> -- for the second rule?
>
> _______________________________________________
> Glasgow-haskell-users mailing list
> Glasgow-haskell-users@haskell.org
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--
Joachim Breitner
  mail@joachim-breitner.de
  http://www.joachim-breitner.de/