# [Template-haskell] newbie question

**Isaac Jones
**
ijones@syntaxpolice.org

*Tue, 08 Jul 2003 17:11:06 -0400*

Greetings,
I'm new to Template Haskell and am trying to work out some examples.
I successfully used this function (makeSum n) to generate a function
that takes n arguments.
\begin{code}
-- |Given a size, n and n expressions, sum them up, so:
-- > $(makeSum 4) 1 2 3 10
-- => 16
-- > $(makeSum 4) 1.0 2.2 3.4 10.1
-- => 16.7
makeSum :: Int -> ExpQ
makeSum n = lam aPats (sumExps aVars)
where
-- create the symbols we'll need:
aPats ::[Pat]
aVars ::[ExpQ]
(aPats,aVars) = genPE "num" n
sumExps :: [ExpQ] -> ExpQ
sumExps (h:t) = [| $h + $(sumExps t) |]
sumExps [] = [| 0 |]
\end{code}
Now I would like to bring a bunch of versions of this function into
scope, something like
>* $(genSummers 10)
*
where genSummers :: Int -> Q [Dec]
and brings into scope:
makeSum0 :: Int
...
makeSum2 :: Int -> Int -> Int
...
makeSum10 :: ...
So this seems to make sense:
>* genSummers n = returnQ [Fun ("makeSum" ++ (show i)) [Clause [] (Normal $(makeSum i)) [] ] | i <- [1..n] ]
*
But actually, I run into a stage restriction. Now I can create a function:
>* makeSumE :: Int -> Exp
*
and say:
>* genSummers n = returnQ [Fun ("makeSum" ++ (show i)) [Clause [] (Normal (makeSumE i)) [] ] | i <- [1..n] ]
*
But how to define makeSumE? I try this:
\begin{code}
makeSumE :: Int -> Exp
makeSumE n = Lam aPats (sumExps aVars)
where
-- create the symbols we'll need:
aPats ::[Pat]
aVars ::[Exp]
(aPats,aVars) = genPE' "num" n
sumExps :: [Exp] -> Exp
sumExps (h:t) = [| $h + $(sumExps t) |] --oops, can't do that
sumExps [] = Lit $ Integer 0
\end{code}
But alas, the commented line is not the right type.
So how do I write genSummers?
peace,
isaac
ps. I implemented the printf and sel examples from the paper with the
real stuff from the GHC6 library, and that code can be found on the
wiki:
http://www.haskell.org/hawiki/TemplateHaskell
Maybe when I come up with a handful of nice examples I can add them to
the haddock for the Language.Haskell.THSyntax