[Template-haskell] re: newbie question
Wed, 09 Jul 2003 10:34:11 -0400
> [Isaac, you need to join the TH list so you can post to it without
> me having to intervene.]
> Your original makeSum looked better.
OK, thats what I was hoping.
> Solution: use the "smart constructors" from the paper. More like
> genSummers n = fun ("makeSum" ++ (show i)) [clause  (normal
> (makeSum i))  ]
Thanks, for some reason I was blind to the combination of using the
"smart constructors" in together with sequenceQ. I thought I had
tried every combination to make the types work out.
Here's my solution below.
-- |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)
-- create the symbols we'll need:
(aPats,aVars) = genPE "num" n
sumExps :: [ExpQ] -> ExpQ
sumExps (h:t) = [| $h + $(sumExps t) |]
sumExps  = [| 0 |]
-- |Bring functions named makeSumX into scope where X is in [1..n]
genSummers :: Int -> Q [Dec]
genSummers n = sequenceQ [fun ("makeSum" ++ (show i)) [clause  (normal (makeSum i))  ] | i <- [1..n] ]
-- Then in a different file, I can say
-- $(genSummers 10)
-- to bring makeSum1, makeSum2, etc into scope