[Haskell-beginners] "Template Haskell and higher order meta programming."

[Song Zhang]

I generally have 2 questions. I am writing some functions to derive
typeclass instances with a package called derive. I just want to get rid of
using Derivation type with a stupid reason. I want use just Name type.

How can I defined a function with type

derivings :: Name -> Name -> Q [Dec]

by using function derive :: Derivation -> Name -> Q [Dec]
so that user can just write derivings ''Eq ''D instead of derive makeEq ''D?

> {-# LANGUAGE TemplateHaskell #-}

> import Data.DeriveTH -- in derive package
> import Language.Haskell.TH
> import qualified Language.Haskell.TH.Syntax as S
> data D = D
> derivings :: Name -> String -> Q Exp
> derivings cla typ = do
>               let makeClassName = mkName $ "make" ++ nameBase cla
>               a <-  [| derive makeClassName (mkName typ) |]
>               return a

> instance S.Lift Name where
>         lift x = varE x

Main> :t derivings ''Eq "D"
derivings ''Eq "D" :: Q Exp

Main> :t $(derivings ''Eq "D")
$(derivings ''Eq "D") :: Q [Dec]

There is no problem to quote twice in other file in order to get the
declaration. Maybe it should called second order meta programming. But the
following has problems:

> derivings'' :: Name -> Name -> Q Exp
> derivings'' cla typ = do
>               let makeClassName = mkName $ "make" ++ nameBase cla
>               a <-  [| derive makeClassName typ |]
>               return a

Main> :t derivings''
derivings'' :: Name -> Name -> Q Exp

Main> :t derivings'' ''Eq ''D
derivings'' ''Eq ''D :: Q Exp

Main> :t $(derivings'' ''Eq ''D)
<interactive>:1:3:
    Illegal variable name: ‘D’
    When splicing a TH expression:
      Data.DeriveTH.derive makeEq (Language.Haskell.TH.Syntax.mkName Main.D)
    In the splice: $(derivings'' ''Eq ''D)

In the first case I used String, it works. however, it is not workiing if I
use D with a Name type. Cannot figure it out.
My second question is that if my return type is Q [Exp] while the [Exp] can
be further expanded into [Dec]. How can I do it to expand [Exp] into [Dec]?
Do we have a function for $ in $(thcode_with_Q_Exp) so that I do not need
to splice each Exp value?

Best wishes
Song
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