March 2009 - template-haskell

New TH features in the HEAD
by Manuel M T Chakravarty 26 Mar '09

26 Mar '09

I added some new language features to Template Haskell in the HEAD repository: * type families, including associated types, * kind annotations, * INLINE and SPECIALISE pragmas, and * bang patterns. Especially, type families were repeatedly requested <http://hackage.haskell.org/trac/ghc/ticket/1673 >. Please give these extensions a spin and let me know of any problems that you may encounter. Unfortunately, I had to change the Language.Haskell.TH API in a way that is not backwards compatible due to the necessary extensions in the Language.Haskell.TH.Syntax. For the moment, this will only affect users of the development version, but I expect these changes to be in the next stable release, 6.12, too. Happy Hacking! Manuel

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generation of type signatures for Accessors
by brian＠lorf.org 25 Mar '09

25 Mar '09

Hey, I'm trying to modify data-accessor-template 'deriveAccessors' to generate type signatures so -Wall doesn't complain. > data P = P { x_::Int, y_::String } > $(deriveAccessors ''P) should generate > x :: Accessor P Int > ... > y :: Accessor P String > ... That seems pretty easy. But > data P a = P { x_::a, y_::a } > $(deriveAccessors ''P) should generate > x :: forall a. T (P a) a > ... > y :: forall a. T (P a) a > ... I was able to write TH to generate code for the first situation, but it failed for the second, and I don't feel confident about writing TH that will work generally. How difficult is it?

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Re: [Haskell-cafe] Proposal for associated type synonyms in Template Haskell
by Sean Leather 19 Mar '09

19 Mar '09

It looks like Manuel Chakravarty is making some progress on this. http://hackage.haskell.org/trac/ghc/ticket/1673#comment:11 Sean On Thu, Jan 15, 2009 at 21:57, Nicolas Frisby wrote: > Any movement on this? > > (I am actually just looking forward to generating kind ascriptions and > having access to the kinds when processing TH.Dec, TH.Type, and such.) > > 2008/11/27 Simon Peyton-Jones <simonpj(a)microsoft.com>: > > I've been away. I hope others will reply to this thread too; whatever > you > > decide will end up in TH indefinitely. I know that Roman is interested > in > > this. > > > > > > > > · You focus just on type families in class declarations (which is > > indeed where associated types started). But I suggest you also allow > them > > at top level, as GHC does using the syntax > > > > type family T a :: * > > > > Indeed, since you propose to add to Dec, that'll happen automatically. > But > > perhaps "AssocTySynKindD" is not a good name. Perhaps "TySynFamilyD"? > > > > > > > > · GHC uses > > > > type instance T [a] = Tree a > > > > as the way to add an equation to the definition of T. So perhaps > > "TySynInstance" rather than "AssocTySynD"? > > > > > > > > · I agree that it'd be good to do data type/newtype families at > the > > same time. Roman needs this. > > > > > > > > · Your proposal for kinds looks fine. > > > > > > > > Simon > > > > > > > > From: haskell-cafe-bounces(a)haskell.org > > [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of José Pedro > Magalhães > > Sent: 11 November 2008 11:11 > > To: Haskell Cafe > > Subject: Re: [Haskell-cafe] Proposal for associated type synonyms in > > Template Haskell > > > > > > > > Hello Thomas, > > > > I see this is a proposal for a partial implementation of #1673 > > (http://hackage.haskell.org/trac/ghc/ticket/1673) Maybe it would be > good if > > the remaining syntax (associated datatypes and type families) would also > be > > defined and implemented in TH. Or maybe there isn't much demand for > this?... > > > > > > Cheers, > > Pedro > > > > On Wed, Nov 5, 2008 at 15:57, Thomas van Noort <thomas(a)cs.ru.nl> wrote: > > > > Hello, > > > > Recently, we released a library on Hackage for generic rewriting (package > > "rewriting" if you are curious). The user of the library is expected to > > define type class instances to enable rewriting on his or her own > datatypes. > > As these instances follow the datatype declarations closely, we tried to > > generate the instances using Template Haskell. Unfortunately, associated > > type synonyms are not yet supported by TH. > > > > After a presentation at the WGP'08, Simon encouraged us to write a > proposal > > about adding associated type synonyms to TH, so that it can be added to > GHC. > > So, here is our proposal. > > > > The TH AST must allow 1) kind declarations of associated type synonyms > > in class declarations and 2) their definitions in instance declarations. > For > > example, > > > > class Foo a where > > type Bar a :: * > > > > instance Foo Int where > > type Bar Int = String > > > > The TH library defines a datatype Dec which contains a constructor for > class > > declarations and instance declarations: > > > > data Dec > > = ... > > | ClassD Cxt Name [Name] [FunDep] [Dec] > > | InstanceD Cxt Type [Dec] > > ... > > > > 1) Associated type synonym kind declarations > > > > We suggest to add a constructor to the Dec type: > > > > ... > > | AssocTySynKindD Name [Name] (Maybe Kind) > > ... > > > > assocTySynKindD :: Name -> [Name] -> Maybe KindQ -> DecQ > > > > The first field is the name of the associated type synonym, the second > field > > is a list of type variables, and the third field is an optional kind. > Since > > kinds are not yet defined in TH, we have to add some kind of kind > definition > > (pun intended): > > > > data Kind > > = StarK > > | ArrowK Kind Kind > > > > type KindQ = Q Kind > > starK :: KindQ > > arrowK :: KindQ -> KindQ -> KindQ > > > > We explicitly choose not to reuse the Type type to define kinds (i.e., > type > > Kind = Type as in GHC) since we think a separation between the two worlds > is > > much clearer to the users of TH. > > > > 2) Associated type synonym definitions > > > > We suggest to add another constructor to the Dec type: > > > > ... > > | AssocTySynD Name [Type] Type > > ... > > > > assocTySynD :: Name -> [TypeQ] -> TypeQ -> DecQ > > > > The first field is the name of the type synonym, the second field is a > list > > of type arguments, and the third field is the body of the type synonym. > > > > We would like to hear your comments to this proposal. > > > > Regards, > > Thomas >

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