Proposal: Lifting to TExpQ - Libraries - Haskell.org

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Re: Proposal: Lifting to TExpQ

Proposal: Lifting to TExpQ

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Emil Axelsson

12 Jan 2018 12 Jan '18

5:47 a.m.

Hi! I wasn't able to find a function like this anywhere: liftT :: Lift a => a -> TExpQ a liftT = unsafeTExpCoerce . lift It seems like a nice addition that would reduce the need for `unsafeTExpCoerce` in user code. Should it be added? / Emil

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Simon Peyton Jones

12 Jan 12 Jan

8:17 a.m.

Indeed. Moreover there is nothing unsafe about this, so I'd like to see this done without using unsafe-anything. To do this, the Lift class would need to change, from class Lift t where lift :: t -> Q Exp to class Lift t where lift :: t -> Q (TExp t) This would take a bit of work, but it looks to me as if it would be a Good Thing. Maybe someone could write up a design? Simon | -----Original Message----- | From: Libraries [mailto:libraries-bounces@haskell.org] On Behalf Of | Emil Axelsson | Sent: 12 January 2018 10:48 | To: Haskell Libraries | Subject: Proposal: Lifting to TExpQ | | Hi! | | I wasn't able to find a function like this anywhere: | | liftT :: Lift a => a -> TExpQ a | liftT = unsafeTExpCoerce . lift | | It seems like a nice addition that would reduce the need for | `unsafeTExpCoerce` in user code. | | Should it be added? | | / Emil | _______________________________________________ | Libraries mailing list | Libraries@haskell.org | https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fmail.h | askell.org%2Fcgi- | bin%2Fmailman%2Flistinfo%2Flibraries&data=02%7C01%7Csimonpj%40microsof | t.com%7C3f68f4551f724d2d8cfa08d559a9f13b%7C72f988bf86f141af91ab2d7cd01 | 1db47%7C1%7C0%7C636513508787847030&sdata=FUvPD77GrLn%2Bkc3ZUwfinQacU8p | s00oXFvfI6VhoD1Q%3D&reserved=0

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Emil Axelsson

13 Jan 13 Jan

5:42 p.m.

Ah, yes. That's more than I could take on now, but sounds like the right way to do lifting. / Emil Den 2018-01-12 kl. 13:17, skrev Simon Peyton Jones:

Indeed. Moreover there is nothing unsafe about this, so I'd like to see this done without using unsafe-anything.

To do this, the Lift class would need to change, from

class Lift t where lift :: t -> Q Exp

to

class Lift t where lift :: t -> Q (TExp t)

This would take a bit of work, but it looks to me as if it would be a Good Thing. Maybe someone could write up a design?

Simon

| -----Original Message----- | From: Libraries [mailto:libraries-bounces@haskell.org] On Behalf Of | Emil Axelsson | Sent: 12 January 2018 10:48 | To: Haskell Libraries | Subject: Proposal: Lifting to TExpQ | | Hi! | | I wasn't able to find a function like this anywhere: | | liftT :: Lift a => a -> TExpQ a | liftT = unsafeTExpCoerce . lift | | It seems like a nice addition that would reduce the need for | `unsafeTExpCoerce` in user code. | | Should it be added? | | / Emil | _______________________________________________ | Libraries mailing list | Libraries@haskell.org | https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fmail.h | askell.org%2Fcgi- | bin%2Fmailman%2Flistinfo%2Flibraries&data=02%7C01%7Csimonpj%40microsof | t.com%7C3f68f4551f724d2d8cfa08d559a9f13b%7C72f988bf86f141af91ab2d7cd01 | 1db47%7C1%7C0%7C636513508787847030&sdata=FUvPD77GrLn%2Bkc3ZUwfinQacU8p | s00oXFvfI6VhoD1Q%3D&reserved=0

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Michael Sloan

6:02 p.m.

Hmm, I'm not sure about changing the Lift class, there are good reasons to leave it the same and good reasons to change as well. There is a lot of precedent for providing safe functions which use unsafe functions (e.g. unsafeDupablePerformIO in bytestring / text). Also, not much of the TH API uses TExp, just typed splices. On the other hand, it does make the method's type stronger and directly suggest more properties. It may encourage people to use typed splices to define manual instances, which I think is good. I propose a somewhat less breaking change, defining the following: class Lift t where typedLift :: t -> Q (TExp t) lift :: Lift t => t -> Q t lift = fmap unType . typedLift The advantage of this is that a lot more code depends on usage of 'lift' than defining instances of 'Lift'. A lot of the code that defines instances of 'Lift' does it via the 'th-lift' package. I believe that only a handful of packages would need to be updated as a result of this change, vs the dozens that would need to be updated by changing the type of 'lift'. Even after this change, I think that since 'TExp' is only useful with typed splices, the 'lift' form would still find much more usage as the more convenient function. -Michael

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Emil Axelsson
Michael Sloan
Simon Peyton Jones