Folks Quite a few people have asked for splices in Template Haskell *types*, and I have finally gotten around to implementing them. So now you can write things like instance Binary $(blah blah) where ... or f :: $(wubble bubble) -> Int as requested, for example, in the message below. Give it a whirl. You need the HEAD; in a day or two you should find binary snapshots if you don't want to build from source. Simon PS: Note that you (still) cannot write a splice in a *binding* position. Thus you can't write f $(blah blah) = e or data T $(blah blah) = MkT Int I don't intend to change this; see the commentary at http://hackage.haskell.org/trac/ghc/ticket/1476 | -----Original Message----- | From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On | Behalf Of Ross Mellgren | Sent: 25 January 2009 19:55 | To: Haskell Cafe | Subject: [Haskell-cafe] Template Haskell very wordy w/r/t Decs and Types | | Hi all, | | I'm writing a small module that exposes a template haskell splice that | takes a (very simplified) C struct definition and builds: | | - A data type definition, | - an instance for Data.Binary.Binary, | - and optionally a pretty print function for it | | However, it seems to do this I have to write a bunch of really ugly | code that builds up the TH data structures "by hand" because quoting | only works with splices for expressions, or so it seems. | | For example, to generate the binary instance I have this code: | | import qualified Language.Haskell.TH as TH | | -- tyname is the name of the data type I've already created, as a | TH.Name | -- tempnames is a list of temporary variable names that are used in | lambda patterns | -- fields is a list of tuples describing each field | -- makeGetExp recursively builds a monadic computation consisting | mostly of Binary.getWord32be >>= \ tempvar -> ... | | binaryInstDec <- liftM (TH.InstanceD [] (TH.AppT (TH.ConT $ | TH.mkName "Data.Binary.Binary") (TH.ConT tyname))) | [d| get = $(makeGetExp (reverse $ zip | fields tempnames) returnExp) | put = undefined |] | | I'd really rather write: | | binaryInstDec <- [d| | instance Binary.Binary $(tyname) where | get = $(makeGetExp (reverse $ zip fields tempnames) | returnExp) | put = undefined |] | | But GHC gives me a syntax error on the tyname splice. The docs seem to | indicate this is the way it is -- that splices in type locations is | plain not implemented. | | My question is whether or not this is just the way it is, and people | writing TH declaration splices tend to have to start manually | assembling a bunch of it, or is there some trick I've missed? Perhaps | even better are there some tricks that people tend to use to make this | less painful? | | I did try using some of the lowercased monadic constructors in | Language.Haskell.TH.Lib but I didn't seem to get anything more succint | out of it.