October 2007 - template-haskell

TupleT type constructor
by Alfonso Acosta 05 Nov '07

05 Nov '07

Hi, I'm a bit confused about TupleT . Is is supposed to be only used when synthesizing types? (i.e. reify will never output such a type)? ~/ghc/compiler/stage2$ ./ghc-inplace --interactive -fth GHCi, version 6.9.20071025: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude> :m +Language.Haskell.TH Prelude Language.Haskell.TH> let l = (1,2) Prelude Language.Haskell.TH> $( do {VarI _ t _ _ <- reify (mkName "l"); runIO $ putStrLn (pprint t); [| 1 |]}) Loading package array-0.1 ... linking ... done. Loading package packedstring-0.1 ... linking ... done. Loading package containers-0.1 ... linking ... done. Loading package pretty-1.0 ... linking ... done. Loading package template-haskell ... linking ... done. Data.Tuple.(,) GHC.Num.Integer GHC.Num.Integer Data.Tuple.(,) GHC.Num.Integer GHC.Num.Integer Data.Tuple.(,) GHC.Num.Integer GHC.Num.Integer Data.Tuple.(,) GHC.Num.Integer GHC.Num.Integer 1 (The four outputs are a known TH bug) >From the definition of "Type" I would have expected to get (TupleT 2 `AppT` ConT "GHC.Num.Integer") `AppT` (ConT "GHC.Num.Integer") Should this be considered a bug or a feature? IMHO it's quite ugly to have two valid representations for the same type in the same AST structure. Best Regards, Fons

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Lift instances of the AST-related types
by Alfonso Acosta 01 Nov '07

01 Nov '07

Hi, It has been suggested a few types in this list [1,2] that it would be helpful to include a Lift instance of Exp, Type, Dec and friends. Having such instances allow to store the AST in a TH-generated structure for later processing during runtime (something curcial in the embedded compiler I'm currently developing). It would be great that the instances were included in the library itself or if (even better) GHC supported automatic deriving of Lift. Until that happens, I have used a modified version Ian's th-lift[3] to automatically generate the instances. Just in case anyone is interested, I attached the patch of th-lift and the instantiation module. Cheers, Fons [1] http://www.haskell.org/pipermail/template-haskell/2007-February/000593.html [2] http://www.haskell.org/pipermail/template-haskell/2004-June/000289.html [3] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/th-lift-0.2

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[Bug?] AST representation of type-synonyms with phantom parameters
by Alfonso Acosta 30 Oct '07

30 Oct '07

Hi, There seems to be a bug in the type representation of type synonyms with phantom parameters === Phantom.hs === module Phantom where -- non-phantom type synonym type List a = [a] type PhantomSyn a = Int data PhantomData a = PhantomData Int ================== ~/ghc/ghc/compiler/stage2$ ./ghc-inplace -fth --interactive /tmp/Phantom.hs GHCi, version 6.9.20071025: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. [1 of 1] Compiling Phantom ( /tmp/Phantom.hs, interpreted ) Ok, modules loaded: Phantom. *Phantom> let {syn = undefined :: List a; phanSyn = undefined :: PhantomSyn a; phanData = undefined :: PhantomData a} *Phantom> :m +Language.Haskell.TH *Phantom Language.Haskell.TH> let showType name = do {VarI _ t _ _ <- reify name ; runIO $ putStrLn (pprint t); [| 1 |]} Loading package array-0.1 ... linking ... done. Loading package packedstring-0.1 ... linking ... done. Loading package containers-0.1 ... linking ... done. Loading package pretty-1.0 ... linking ... done. Loading package template-haskell ... linking ... done. *Phantom Language.Haskell.TH> $(showType (mkName "syn")) forall a_0 . Phantom.List a_0 forall a_0 . Phantom.List a_0 forall a_0 . Phantom.List a_0 forall a_0 . Phantom.List a_0 1 *Phantom Language.Haskell.TH> $(showType (mkName "phanSyn")) Phantom.PhantomSyn GHC.Prim.Any Phantom.PhantomSyn GHC.Prim.Any Phantom.PhantomSyn GHC.Prim.Any Phantom.PhantomSyn GHC.Prim.Any 1 *Phantom Language.Haskell.TH> $(showType (mkName "phanData")) forall a_0 . Phantom.PhantomData a_0 forall a_0 . Phantom.PhantomData a_0 forall a_0 . Phantom.PhantomData a_0 forall a_0 . Phantom.PhantomData a_0 1 The output makes sense for "forall a_0 . Phantom.List a_0" and "forall a_0 . Phantom.PhantomData a_0" but why "Phantom.PhantomSyn GHC.Prim.Any"? (just for information purposes, ghc 6.6 uses "()" instead of "Any") Shouldn't it be "forall a_0 . Phantom.PhantomSyn a_0"? Best Regards, Fons PS: BTW, $(showType 'phanSyn) works fine in ghci6.6 but gives a Stage error in HEAD. That's why I used mkName "phanSyn" Is that another bug? (I couldn't find a related ticket in GHC's trac)

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What happened to currentLoc ?
by Alfonso Acosta 25 Oct '07

25 Oct '07

Hi, I've been trying to improve error reporting in my TH-based embedded compiler. I sometimes build data structures using splices. Those structures can later cause an error at runtime which, to be more informative, should give the line and module in which the splice was executed (i.e. tell to what structure the error refering by telling where it was exactly created) I have just read about currentLoc in the "Notes on Template Haskell 2" document. currentLoc :: Q (FilePath, Int) -- Returns the location of the top-level splice being executed. Was it ever implemented? Are there any plans to do it? The path to the module is not really important (it should be enough with currentModule) but the line information can be really helpful. Thanks, Fons

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Defining a function and using it in a splice within the same module
by Alfonso Acosta 25 Oct '07

25 Oct '07

Hi, The user guide from latest GHC release makes clear that doing what I wrote in the subject of this message is not possible: >From 7.6: "You can only run a function at compile time if it is imported from another module. That is, you can't define a function in a module, and call it from within a splice in the same module." Unfortunately I have encountered this problem more times than I'd like while using TH to implement a DSL. My usual solution is to generate a function to process the extra arguments which cannot be passed to the splice (i.e. push the arguments out of the splice so that I can use functions defined in the same module). Nevertheless, I'd like to know if there are any chances of seeing this problem solved anytime soon. Thanks, Alfonso

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Any chances of having TH included in Haskell II?
by Alfonso Acosta 04 Oct '07

04 Oct '07

Hi, I recently developed an embedded DSL compiler making use of TH (and my plan is to continue with its development). However, my employers are naturally concerned about the long-term support of Template Haskell. I've been trying to gather some information about it, but I don't seem to find any opened tickets in haskell-prime. Can anyone confirm if there are any plans of including Template Haskell in the next standard and/or any information regarding long-term support? Best Regards, Alfonso Acosta

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