SevenThunders wrote:
test.hs:14:11: No instance for (PrintfType (t t1)) arising from use of `printf' at test.hs:14:11-16 Probable fix: add an instance declaration for (PrintfType (t t1)) In the result of a 'do' expression: printf "%14.7g" u In the definition of `test': test = do printf "%14.7g" u Failed, modules loaded: none.
The problem here appears to be the monomorphism restriction. This means that all declarations without arguments get a monomorphic type, a type without variables. The type of 'test' would be 'PrintfType r => r', but the compiler does not allow this type. The solution is to either: - make the type explicit by adding a type declaration, "test :: IO ()" or "Test :: PrintfType r => r" - make test local in a context where it is only used as an IO action, for example: > main :: IO () > main = do {... ; test ; ...} > where test = printf "%14.7g" 3.14 - add a parameter to test Twan