RE: [Template-haskell] Re: [Haskell] Proposal: Allow "\=" for field update in record update syntax
To have what you want takes a syntactic extension to Haskell; TH requires '$' to indicate splices. You'd have to write something like $( [| name |] =:= val ) instead of name := val where (=:=) :: ExpQ -> a -> ExpQ You can do that today, but I guess you won't find it as convenient. It's very hard to know where to stop when adding syntax; and the record design space is particularly complicated (there have been many proposals, all with different relative merits). And defining new syntax often makes some existing programs stop working; in this case, ones that use ":=" as a data constructor. Simon | -----Original Message----- | From: S. Alexander Jacobson [mailto:alex@alexjacobson.com] | Sent: 07 March 2005 16:46 | To: Simon Peyton-Jones | Cc: template-haskell@haskell.org; Sander Evers | Subject: RE: [Template-haskell] Re: [Haskell] Proposal: Allow "\=" for field update in record update | syntax | | Most generally, I use records all over the place in my code (is this | bad style?), and the current syntax for record update is really | unwieldy. | | I'd like some way to have operators like (:=) and (:\=) for record | update. I don't yet know TemplateHaskell so I don't know how you | define functions in it, but I would like some way to write: | | name := val = \rec-> rec { name = val} | name :\= fn = \rec -> rec { name = fn $ name rec} | | And I would like to be able to use these operators as Sander Evers | showed: | | data Country = Country {capital :: City, ...} | data City = City {inhabitants :: Int, ...} | | birth_in_capital :: Country -> Country | birth_in_capital = capital :\= (inhabitants :\= (+1)) | | It would be even nicer if we could define the precedence of these | operators so that we could write the above | | birth_in_capital = capital :\= inhabitants :\= (+1) | | Though since haskell is usually prefix, perhaps it would be best to be | able to write it: | | birth_in_capital = (+1) :\= inhabitants :\= capital | | Note: It is entirely possible that this is a Haskell rather than a | template haskell issue. I don't know the border between the projects. | | -Alex- | | ______________________________________________________________ | S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com | | | | On Mon, 7 Mar 2005, Simon Peyton-Jones wrote: | | > What exactly is it that TH doesn't support, that you want? | > | > Simon | > | > | -----Original Message----- | > | From: template-haskell-bounces@haskell.org | > [mailto:template-haskell-bounces@haskell.org] On | > | Behalf Of S. Alexander Jacobson | > | Sent: 28 February 2005 10:32 | > | To: Sander Evers; template-haskell@haskell.org | > | Subject: [Template-haskell] Re: [Haskell] Proposal: Allow "\=" for | > field update in record update syntax | > | | > | Since operators can't start with a ':', is there an in-principle | > | reason TH does not support : operators? The syntax below would be | > | really nice to have. | > | | > | | > | | > | On Fri, 18 Feb 2005, Sander Evers wrote: | > | | > | > | > | >> Then we could have e.g. (:=) and (:\=) and end up with a | > | >> syntax that looks like this: | > | >> | > | >> rec // field :\= fn . field2 := val | > | >> | > | >> And that is BETTER than the Haskell syntax that I originally | > proposed. | > | > | > | > This syntax is also nice for composition of update functions, which | > is useful | > | > with nested records: | > | > | > | > data Country = Country {capital :: City, ...} | > | > data City = City {inhabitants :: Int, ...} | > | > | > | > birth_in_capital :: Country -> Country | > | > birth_in_capital = capital :\= (inhabitants :\= (+1)) | > | > | > | > This also shows that it would be useful to make :\= associate to the | > right. | > | > | > | | > | ______________________________________________________________ | > | S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com | > | _______________________________________________ | > | template-haskell mailing list | > | template-haskell@haskell.org | > | http://www.haskell.org/mailman/listinfo/template-haskell | >
Simon Peyton-Jones wrote:
To have what you want takes a syntactic extension to Haskell; TH requires '$' to indicate splices. You'd have to write something like
$( [| name |] =:= val )
instead of
name := val
where (=:=) :: ExpQ -> a -> ExpQ
You can do that today, but I guess you won't find it as convenient.
It's very hard to know where to stop when adding syntax; and the record design space is particularly complicated (there have been many proposals, all with different relative merits). And defining new syntax often makes some existing programs stop working; in this case, ones that use ":=" as a data constructor.
Simon
Surely thats an argument for template-haskell to support adding some syntax, as everyone can define their own record short-hand... or in fact a consensus can develop through a standard library. Can you use splices an infix position? name $=:= val name $`eq` val name `$eq` val Are any of the above a possibility? Keean.
| -----Original Message----- | From: S. Alexander Jacobson [mailto:alex@alexjacobson.com] | Sent: 07 March 2005 16:46 | To: Simon Peyton-Jones | Cc: template-haskell@haskell.org; Sander Evers | Subject: RE: [Template-haskell] Re: [Haskell] Proposal: Allow "\=" for field update in record update | syntax | | Most generally, I use records all over the place in my code (is this | bad style?), and the current syntax for record update is really | unwieldy. | | I'd like some way to have operators like (:=) and (:\=) for record | update. I don't yet know TemplateHaskell so I don't know how you | define functions in it, but I would like some way to write: | | name := val = \rec-> rec { name = val} | name :\= fn = \rec -> rec { name = fn $ name rec} | | And I would like to be able to use these operators as Sander Evers | showed: | | data Country = Country {capital :: City, ...} | data City = City {inhabitants :: Int, ...} | | birth_in_capital :: Country -> Country | birth_in_capital = capital :\= (inhabitants :\= (+1)) | | It would be even nicer if we could define the precedence of these | operators so that we could write the above | | birth_in_capital = capital :\= inhabitants :\= (+1) | | Though since haskell is usually prefix, perhaps it would be best to be | able to write it: | | birth_in_capital = (+1) :\= inhabitants :\= capital | | Note: It is entirely possible that this is a Haskell rather than a | template haskell issue. I don't know the border between the projects. | | -Alex- | | ______________________________________________________________ | S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com | | | | On Mon, 7 Mar 2005, Simon Peyton-Jones wrote: | | > What exactly is it that TH doesn't support, that you want? | > | > Simon | > | > | -----Original Message----- | > | From: template-haskell-bounces@haskell.org | > [mailto:template-haskell-bounces@haskell.org] On | > | Behalf Of S. Alexander Jacobson | > | Sent: 28 February 2005 10:32 | > | To: Sander Evers; template-haskell@haskell.org | > | Subject: [Template-haskell] Re: [Haskell] Proposal: Allow "\=" for | > field update in record update syntax | > | | > | Since operators can't start with a ':', is there an in-principle | > | reason TH does not support : operators? The syntax below would be | > | really nice to have. | > | | > | | > | | > | On Fri, 18 Feb 2005, Sander Evers wrote: | > | | > | > | > | >> Then we could have e.g. (:=) and (:\=) and end up with a | > | >> syntax that looks like this: | > | >> | > | >> rec // field :\= fn . field2 := val | > | >> | > | >> And that is BETTER than the Haskell syntax that I originally | > proposed. | > | > | > | > This syntax is also nice for composition of update functions, which | > is useful | > | > with nested records: | > | > | > | > data Country = Country {capital :: City, ...} | > | > data City = City {inhabitants :: Int, ...} | > | > | > | > birth_in_capital :: Country -> Country | > | > birth_in_capital = capital :\= (inhabitants :\= (+1)) | > | > | > | > This also shows that it would be useful to make :\= associate to the | > right. | > | > | > | | > | ______________________________________________________________ | > | S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com | > | _______________________________________________ | > | template-haskell mailing list | > | template-haskell@haskell.org | > | http://www.haskell.org/mailman/listinfo/template-haskell | >
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participants (2)
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Keean Schupke -
Simon Peyton-Jones