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
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
