Hi Sylvain, sylvain wrote:
the following code snippet
data Test = A Int instance Show Test main = print (A 1)
leads to:
*Main> main *** Exception: stack overflow
Since "instance Show Test" does not specify how method "show" should be implemented for datatype Test, "show = undefined" is assumed. Now, print calls show to create a string represention of (A 1), which produces the error message. Interestingly, undefined gives a better error message, when called directly: Prelude> undefined *** Exception: Prelude.undefined I don't know why the behaviour of undefined differs from the behaviour of unbound methods. If you want to correct the above program, you could either bind show to something, as in instance Show Test where show (A n) = "(A " ++ show n ++ ")" or you could use ask the compiler to derive an instance for you, as in data Test = Show Int deriving Show Tillmann