On Sunday 14 May 2006 03:00 pm, SevenThunders wrote:
Thanks that helps a bit. The realToFrac type coercion works, but ultimately it seems that printf won't play nice. Consider this simple haskell code
module Test where import IO -- import Data.Array.Storable import Text.Printf import Foreign.C.Types (CInt, CDouble )
y :: CDouble y = 5.2 u = realToFrac(y) test = do printf "%14.7g" u
Try: y :: CDouble y = 5.2 u :: Double u = realToFrac(y) test = do printf "%14.7g" u The root problem seems to be that GHC isn't sure what type 'u' has unless you fix it with a type signature because 'realToFrac' has a polymorphic type. I'm not sure why it works when you type it at the interpreter.
Compiling it into GHCi I get the error, test.hs:13:11: No instance for (PrintfType (t t1)) arising from use of `printf' at test.hs:13:11-16 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.
If I replace the printf by a standard print there is no problem. Also if I comment out the line with test = ... , it will load (compile) and I can then type in the interpreter printf "%14.7g" u and it works!
but printf "%14.7g" y
fails because y is of type CDouble. At this point I think I'm giving up on the formatted printing in Haskell. It's just too persnickety. I guess I'll just call printf via C and see what happens.
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