Am Mittwoch 11 November 2009 23:50:21 schrieb Thomas DuBuisson:
Like paolino, I did a couple tests and found:
So the read for that does not work... but surprisingly...
data TreeX = Leaf Int | NotLeaf deriving (Show, Read)
Dropping the Int from the second constructor works (ignore the constructor names, they are just place-holders).
--- OTHER TESTS --- 1) data TreeX = Leaf | NotLeaf 5 deriving (Show, Read) Another unfortunate bug is that reversing the constructors (having Leaf as a nullary constructor and NotLeaf taking an Int) causes compilation to fail (using jhc-0.7.2-0).
2) data TreeX = Leaf Int | NotLeaf Int | OoopsLeaf deriving (Show, Read)
Works fine - notice it ends with a nullary constructor... Hypothesis 1: All working Read derivations have data declarations with a nullary constructor at the end.
Must be something like that, it also dies badly reading Either (but reading integers works): module Main where a, n :: Either Int Char a = Right 'a' n = Left 4 sa = "Right 'a'" sn = "Left 4" main :: IO () main = do putStrLn "Showing:" print (sa == show a) print (sn == show n) putStrLn "Reading:" print (read "123" :: Integer) print (a == read sa) print (n == read sn) results in $ ./veither Showing: True True Reading: 123 veither_code.c:3642: case fell off $ jhc --version jhc 0.7.2 (0.7.2-0) compiled by ghc-6.10 on a i386 running linux
Cheers, Thomas