Sat, 03 Feb 2001 23:34:37 +1100, Manuel M. T. Chakravarty <chak@cse.unsw.edu.au> pisze:
fromIntegral :: (Integral a, Num b) => a -> b fromRealFrac :: (RealFrac a, Fractional b) => a -> b
(`fromRealFrac' is also defined in Figure 7.)
However, in Appendix A ``Standard Prelude'', we have
realToFrac :: (Real a, Fractional b) => a -> b realToFrac = fromRational . toRational
instead of a definition for `fromRealFrac'.
Both GHC and Hugs go by Appendix A. What was the original intention?
I don't know. I always assumed that it's realToFrac. It yields a quite consistent picture. Classes provide conversions to and from two "universal" types: toInteger (from Integral), fromInteger (to Num), toRational (from Real), fromRational (to Fractional). There are also conversions with both ends overloaded, defined as compositions of appropriate conversions going through an universal type. One of them converts from Real to Fractional and is thus called realToFrac. The other converts from Integral to Num, and since Num in the context of numeric types does not mean anything (all numeric types are Num), it is omitted from the name fromIntegral. Classes for these four conversions are chosen in a natural way. toInteger and toRational require that appropriate types contains *at most* integral and real values. fromInteger and fromRational require that the given type contains *at least* integral and fractional values (and all numeric types contain at least integral values). It follows that for most injections (except between one complex type to another) either of fromIntegral or realToFrac works. If both work, they give the same result, although fromIntegral should be slightly more efficient. Narrowing the range or precision is not considered a failure to be an injection. fromRealFrac taking RealFrac as an argument does not make much sense: it is not necessary to have fractional numbers to be convertible to Rational, because integral types will do too. -- __("< Marcin Kowalczyk * qrczak@knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK