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Main.hs

{-# LANGUAGE TemplateHaskell #-}

import Language.Haskell.TH
import Control.Monad

data Cons a b = Cons a b
data Nil = Nil

nestedTuple n = do
    xs <- replicateM n (newName "x")
    return $ LamE [foldr (\v prev -> ConP 'Cons [VarP v,prev]) (ConP 'Nil []) xs]
                (TupE $ map VarE xs)

Load Main into ghci:

*Main> :set -XTemplateHaskell
*Main> :set -ddump-splices
*Main> :t $(nestedTuple 3)
<interactive>:1:2:
    <interactive>:1:2-14: Splicing expression
        nestedTuple 3
      ======>
        \ Cons x[aYQ] Cons x[aYR] Cons x[aYS] Nil
            -> (x[aYQ], x[aYR], x[aYS])
    In the expression: $(nestedTuple 3)
$(nestedTuple 3) :: Cons t (Cons t1 (Cons t2 Nil)) -> (t, t1, t2)

However the inferred type and behavior of the function suggests that the Cons constructors are associated to the right.

Trac metadata

Trac field	Value
Version	6.12.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	vogt.adam@gmail.com
Operating system
Architecture