誰かに答えてもらいたかったのですが、わかりました。これが私が本当に素早く作った解決策です:
module Tuples (updateTuple) where
import Language.Haskell.TH
updateTuple :: Int -> [Int] -> Q Exp
updateTuple len ixs = do
ixfns <- mapM (newIxFunName . (+1)) ixs
ixvns <- mapM newIxVarName [1..len]
let baseVals = map VarE ixvns
modVals = foldr applyFun baseVals $ ixs `zip` ixfns
return . LamE [matchTuple ixfns, matchTuple ixvns] $ TupE modVals
where
matchTuple = TupP . map VarP
newIxFunName = newIndexedName "fun"
newIxVarName = newIndexedName "var"
newIndexedName prefix = newName . (prefix ++) . show
applyFun (ix, fn) = modifyElem ix $ AppE $ VarE fn
modifyElem :: Int -> (a -> a) -> [a] -> [a]
modifyElem 0 f (x:xs) = f x : xs
modifyElem n f (x:xs) = x : modifyElem (n - 1) f xs
modifyElem n _ [] = error $ "index " ++ show n ++ " out of bounds"
使用例:
{-# LANGUAGE TemplateHaskell #-}
module Main where
import Tuples
main :: IO ()
main = print $ $(updateTuple 5 [0, 2, 4])
(\ _ -> 'a', (*2), \ _ -> 42)
(1, 2, 3, 'b', 'c')
コンパイル (生成されたコードを表示するため):
$ ghc -ddump-splices -fforce-recomp main.hs
[1 of 2] Compiling Tuples ( Tuples.hs, Tuples.o )
[2 of 2] Compiling Main ( main.hs, main.o )
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Loading package pretty-1.1.1.0 ... linking ... done.
Loading package array-0.4.0.0 ... linking ... done.
Loading package deepseq-1.3.0.0 ... linking ... done.
Loading package containers-0.4.2.1 ... linking ... done.
Loading package template-haskell ... linking ... done.
main.hs:6:18-40: Splicing expression
updateTuple 5 [0, 2, 4]
======>
\ (fun1_a1Cl, fun3_a1Cm, fun5_a1Cn)
(var1_a1Co, var2_a1Cp, var3_a1Cq, var4_a1Cr, var5_a1Cs)
-> (fun1_a1Cl var1_a1Co, var2_a1Cp, fun3_a1Cm var3_a1Cq,
var4_a1Cr, fun5_a1Cn var5_a1Cs)
Linking main ...
出力:
$ ./main
('a',2,6,'b',42)
EDIT:ラムダの関数が変数と同じインデックスを使用するようにしました。その方が理にかなっています。