Fwd: Availability of Coercible Instances at TH Runtime

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Travis Whitaker

4 May 2018 4 May '18

7:44 a.m.

Given that Coercible instances are Deeply Magical, perhaps I'm being a bit naive here, but I recently tried to write a TH function that can check if one type is a newtype of another (or a newtype of a newtype of another, etc). coercibleToFrom :: Type -> Type -> Q Bool coercibleToFrom tx ty = (&&) <$> isInstance ''Coercible [tx, ty] <*> isInstance ''Coercible [ty, tx] If this worked as I'd hoped, I'm almost certain checking reflexively is redundant. However, I can't seem to get reifyInstances to ever return an InstanceDec for a Coercible instance. Given that these instances are generated on the fly by the typechecker, there's no way to make them available at TH runtime, correct? And, given that, is there an easy way to find out with TH whether not I'll be able to use coerce without taking all the Decs apart to hunt for NewtypeD? Travis

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Richard Eisenberg

4 May 4 May

6:01 p.m.

New subject: Availability of Coercible Instances at TH Runtime

I don't think there's an easy way to do this. We could imagine extending Quasi to have a method to check for coercibility, but I don't think there's a way to do this in the current TH API. Sorry! Richard

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On May 4, 2018, at 3:44 AM, Travis Whitaker wrote:

Given that Coercible instances are Deeply Magical, perhaps I'm being a bit naive here, but I recently tried to write a TH function that can check if one type is a newtype of another (or a newtype of a newtype of another, etc).

coercibleToFrom :: Type -> Type -> Q Bool coercibleToFrom tx ty = (&&) <$> isInstance ''Coercible [tx, ty] <*> isInstance ''Coercible [ty, tx]

If this worked as I'd hoped, I'm almost certain checking reflexively is redundant. However, I can't seem to get reifyInstances to ever return an InstanceDec for a Coercible instance. Given that these instances are generated on the fly by the typechecker, there's no way to make them available at TH runtime, correct? And, given that, is there an easy way to find out with TH whether not I'll be able to use coerce without taking all the Decs apart to hunt for NewtypeD?

Travis

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Travis Whitaker

5 May 5 May

2:01 a.m.

New subject: Availability of Coercible Instances at TH Runtime

A workaround that's suitable for what I'm after is defining a class like this for some fixed T. instance (Coercible a T) => C a I'm curious what it'd take to add a qReifyCoercible method; does the renamer know anything about Coercible? Thanks, Travis On Fri, May 4, 2018 at 11:01 AM, Richard Eisenberg wrote:

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I don't think there's an easy way to do this. We could imagine extending Quasi to have a method to check for coercibility, but I don't think there's a way to do this in the current TH API. Sorry!

Richard

On May 4, 2018, at 3:44 AM, Travis Whitaker wrote:

Given that Coercible instances are Deeply Magical, perhaps I'm being a bit naive here, but I recently tried to write a TH function that can check if one type is a newtype of another (or a newtype of a newtype of another, etc).

coercibleToFrom :: Type -> Type -> Q Bool coercibleToFrom tx ty = (&&) <$> isInstance ''Coercible [tx, ty] <*> isInstance ''Coercible [ty, tx]

If this worked as I'd hoped, I'm almost certain checking reflexively is redundant. However, I can't seem to get reifyInstances to ever return an InstanceDec for a Coercible instance. Given that these instances are generated on the fly by the typechecker, there's no way to make them available at TH runtime, correct? And, given that, is there an easy way to find out with TH whether not I'll be able to use coerce without taking all the Decs apart to hunt for NewtypeD?

Travis

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Richard Eisenberg

7 May 7 May

2:39 a.m.

New subject: Availability of Coercible Instances at TH Runtime

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On May 4, 2018, at 10:01 PM, Travis Whitaker wrote:

I'm curious what it'd take to add a qReifyCoercible method; does the renamer know anything about Coercible?

No, it doesn't, but the Q monad is actually a wrapper around TcM, the type-checker monad. I don't think it would be all that hard to do this. Could be a suitable project for someone new to GHC hacking... Richard

David Kraeutmann

4:37 a.m.

New subject: Availability of Coercible Instances at TH Runtime

Tangentially related, but wasn't there a plan at some point to integrate TH more tightly with GHC? On 5/7/2018 4:39 AM, Richard Eisenberg wrote:

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No, it doesn't, but the Q monad is actually a wrapper around TcM, the type-checker monad. I don't think it would be all that hard to do this. Could be a suitable project for someone new to GHC hacking...

Richard Eisenberg

2:14 p.m.

New subject: Availability of Coercible Instances at TH Runtime

Yes: https://ghc.haskell.org/trac/ghc/wiki/TemplateHaskell/Introspective I think this may be waiting for the trees-that-grow work to be completed, and as far as I know, no one is actively working on this. But I still think it would be a Good Thing. Richard

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On May 7, 2018, at 12:37 AM, David Kraeutmann wrote:

Tangentially related, but wasn't there a plan at some point to integrate TH more tightly with GHC?

On 5/7/2018 4:39 AM, Richard Eisenberg wrote:

...
No, it doesn't, but the Q monad is actually a wrapper around TcM, the type-checker monad. I don't think it would be all that hard to do this. Could be a suitable project for someone new to GHC hacking...

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Shayan Najd

6:31 p.m.

New subject: Availability of Coercible Instances at TH Runtime

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I think this may be waiting for the trees-that-grow work to be completed, and as far as I know, no one is actively working on this.

We've just got a new Google SoC project accepted to push this forward :) /Shayan On Mon, May 7, 2018 at 3:14 PM, Richard Eisenberg wrote:

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Yes: https://ghc.haskell.org/trac/ghc/wiki/TemplateHaskell/Introspective

I think this may be waiting for the trees-that-grow work to be completed, and as far as I know, no one is actively working on this. But I still think it would be a Good Thing.

Richard

...
On May 7, 2018, at 12:37 AM, David Kraeutmann wrote:

Tangentially related, but wasn't there a plan at some point to integrate TH more tightly with GHC?

On 5/7/2018 4:39 AM, Richard Eisenberg wrote:

...
No, it doesn't, but the Q monad is actually a wrapper around TcM, the type-checker monad. I don't think it would be all that hard to do this. Could be a suitable project for someone new to GHC hacking...

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David Kraeutmann
Richard Eisenberg
Shayan Najd
Travis Whitaker