[Haskell-cafe] Re: pi
ok at cs.otago.ac.nz
Wed Oct 10 21:39:43 EDT 2007
On 11 Oct 2007, at 1:34 pm, Dan Weston wrote:
> Actually, [pi] is a constant: piDecimalExpansion :: String.
No, that's another constant.
> A translation from piDecimalExpansion :: String to pi :: Floating a
> => a is already well defined via read :: Read a => String -> a
Wrong. piDecimalExpansion would be infinite. pi is, after all,
a transcendental number. It can be computed incrementally by a
finite algorithm, true. The problem is that read has to read
*all the way to the end*, and there is no end. (More precisely,
either to the end of the string or to the first character that is
not part of a floating point literal.)
> Any definition of pi in the Floating class that differs from (read
> piDecimalExpansion) is erroneous.
In effect, you are proposing that the only non-erroneous
definition of pi is bottom.
I don't think that is very helpful.
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