Template Haskell examples

Foo.hs

{-# LANGUAGE TemplateHaskell, QuasiQuotes #-}

module Foo where

import MyTH

foo x = x + 1

$(describe "dfoo" 'foo)  -- defines the function "dfoo"

$(describe "dInt" ''Int)

$(describe "dBool" ''Bool)

$(describe "dplus" '(+))

$(describe "dnum" ''Num)

MyTH.hs

module MyTH where

import Language.Haskell.TH

describe :: String -> Name -> Q [Dec]
describe fname name = do
    i <- reify name
    let what = 
          case i of
            ClassI _ _ -> "class with visible instances"
            ClassOpI _ _ _ _ -> "class method"
            TyConI _         -> "plain type constructor"
            FamilyI _ _      -> "type or data family"
            DataConI _ _ _ _ -> "data constructor"
            VarI _ _ _ _     -> "value variable"
            TyVarI _ _       -> "type variable"
    let val = nameBase name ++ " is a " ++ what
        clause = Clause [] (NormalB $ LitE $ StringL val) []
    return $ [ FunD (mkName fname) [clause] ]