Why not use:
http://hackage.haskell.org/package/ChristmasTree
which supports even infix constructors and runs in linear time,
Doaitse Swierstra
On 10 Mar 2014, at 14:11 , Semen Trygubenko / Семен Тригубенко
Dear Haskell-cafe,
When deriving (Read), only values can be read in. If one wants to be able to read in constructors, too, is there an easy way out? E.g., the code below works, but the extra book-keeping
f "A" = A ...
is unpleasant — perhaps there's a simpler solution?
{-# LANGUAGE FlexibleInstances #-}
data D = A Int | B Int deriving (Show,Read)
instance Read (Int -> D) where readsPrec = \_ s -> [(f s,"")] where f "A" = A f "B" = B f x = error $ "Invalid constructor " ++ x
main = do let x = read "A 1" :: D print x let g s = read s :: (Int -> D) print $ g "B" 2 print $ g "C" 3
Many thanks in advance, Semen
-- Семен Тригубенко http://trygub.com _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe