[Haskell-cafe] Re: Scraping boilerplate deriving?

[Erik Hesselink]

Yes, if you use template haskell, all top level functions and values
have to be defined before you use them.

Erik

On Tue, Sep 14, 2010 at 18:11, Kevin Jardine <kevinjardine at gmail.com> wrote:
> Hmm - It seems to work if the code is defined before my main function
> and not after it.
>
> Does this have to do with TH being part of the compile process and so
> the order matters?
>
> Kevin
>
> On Sep 14, 6:03 pm, Kevin Jardine <kevinjard... at gmail.com> wrote:
>> Thanks Serguey!
>>
>> The library code compiles, but when I try to use it in client code:
>>
>> a. I get:
>>
>> Not in scope: type constructor or class 'A'
>>
>> and even stranger,
>>
>> b. GHC cannot find any of my code after the
>>
>> $(mkNewType "A")
>>
>> and claims that all the functions I defined there are also not in
>> scope.
>>
>> Any ideas?
>>
>> The CPP solution works but Template Haskell is definitely cooler, so
>> it would be great to get this to work!
>>
>> Kevin
>>
>> On Sep 14, 2:29 pm,  Zefirov <sergu... at gmail.com> wrote:
>>
>> > 2010/9/14 Kevin Jardine <kevinjard... at gmail.com>:
>>
>> > > I would like to use some macro system (perhaps Template Haskell?) to
>> > > reduce this to something like
>>
>> > > defObj MyType
>>
>> > > I've read through some Template Haskell documentation and examples,
>> > > but I find it intimidatingly hard to follow. Does anyone has some code
>> > > suggestions or pointers to something similar?
>>
>> > The solutions first:
>> > -------------------------------------------------
>> > {-# LANGUAGE TemplateHaskell #-}
>>
>> > module T(mkNewType) where
>>
>> > import Language.Haskell.TH
>>
>> > decls = [d|newtype TempDecl = TempDecl Int deriving (Eq,Ord,Show)|]
>> > decl = do
>> >         [d] <- decls
>> >         runIO $ print d -- just to show inetrnals
>> >         return d
>>
>> > mkNewType :: String -> Q [Dec]
>> > mkNewType n = do
>> >         d <- decl
>> >         let name = mkName n
>> >         return $ (\x -> [x]) $ case d of
>> >                 (NewtypeD cxt _ argvars (NormalC _ args) derivings) ->
>> >                         NewtypeD cxt name argvars (NormalC name args) derivings
>> > --------------------------------------
>> > I took perfectly valid declaration, dissected it using case analysis
>> > and changed relevant parts.
>>
>> > And an example client:
>> > -------------------------------------
>> > {-# LANGUAGE TemplateHaskell #-}
>>
>> > import T
>>
>> > $(mkNewType "A")
>> > -------------------------------------
>> > It all work together.
>>
>> > I studied how to use Template Haskell that way: I obtained
>> > declarations of what I need, printed them and looked through
>> > documentation for relevant data types and constructors. It's not
>> > harder that any other library in Haskell, actually.
>> > _______________________________________________
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>>
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