Template Haskell...

MR K P SCHUPKE k.schupke at imperial.ac.uk
Fri Oct 31 13:23:39 EST 2003

With reference to the future of template haskell - it would be nice if you could express the

stringType :: String -> Q [Dec]
stringType s = do
   x <- gensym "x"
   y <- gensym "y"
   return [
      Newtype [] s [] (Constr s [(Strict,Tcon (TconName "String"))]) [],
      Proto ("show"++s) (Tapp (Tapp (Tcon Arrow) (Tcon (TconName s))) (Tcon (TconName "ShowS"))),
      Fun ("show"++s) [Clause [Pcon s [Pvar x]]
         (Normal (App (Var "showString") (Var x))) []],
      Instance [] (Tapp (Tcon (TconName "Show")) (Tcon (TconName s))) [
         Fun "showsPrec" [Clause [Pwild,Pvar x]
            (Normal (App (Var ("show"++s)) (Var x))) []]],
      Instance [] (Tapp (Tcon (TconName "Eq")) (Tcon (TconName s))) [
         Fun "==" [Clause [Pcon s [Pvar x],Pcon s [Pvar y]]
            (Normal (App (App (Var "==") (Var x)) (Var y))) []]] ]


stringType :: String -> Q [Dec]
stringType s = [|
	newtype $s = $s String
	show$s :: $s -> ShowS
	show$s ($s x) = showString x
	instance Show $s where
		showsPrec _ x = show$s x
	instance Eq $s where
		($s x) ($s y) = (x==y)

Maybe with a more specific type like :

stringType :: TypeName -> Q [Dec]

where TypeName would enforce the first letter must be capital rule.
NOTE: to work this would require show$s to crate a function name from
show prepended to the type name...

Why is this not possible with the current template-haskell, and would
this be possible with the proposed extensions (if not, why not, as the
above seems very easy to read?)

	Keead Schupke.

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