The issue here, I guess, is that this behavior is inconsistent. While the default implementation for (**) would produce NaNs and such, the implementation for Float and Double overrides this behavior:
(-3 :: Float) ** (-5) -4.115226e-3 (-3 :: Double) ** (-5) -4.11522633744856e-3
Which is in accordance to what other languages such as C and Python do (and also to what I would expect): C float b = -3.0; float e = -5.0; printf("%f\n", powf(b, e)); -0.004115 python
-3.0 ** -5.0 -0.00411522633744856
The Haskell Report is not clear (or at least I was not able to find a clearer explanation) about this: 6.4.3 Exponentiation and Logarithms The one-argument exponential function exp and the logarithm function log act on floating-point numbers and use base e. logBase a x returns the logarithm of x in base a. sqrt returns the principal square root of a floating-point number. There are three two-argument exponentiation operations: (^) raises any number to a nonnegative integer power, (^^) raises a fractional number to any integer power, and (⋆⋆)takes two floating-point arguments. The value of x^0 or x^^0 is 1 for any x, including zero; 0⋆⋆y is 1 if y is 1, and 0 otherwise. Regards, Emilio On Wed, May 19, 2021 at 12:29 PM Henning Thielemann < lemming@henning-thielemann.de> wrote:
On Wed, 19 May 2021, Viktor Dukhovni wrote:
On Wed, May 19, 2021 at 11:50:22AM -0300, Fabrício Olivetti de França wrote:
So I was wondering, is there any reason to have this (potentially dangerous) default implementation on base?
The behaviour for negative inputs seems to be correct:
λ> log(-2) NaN λ> exp(log(-2)) NaN λ> exp(log(-2) * (-2)) NaN λ> log(0) -Infinity
were you expecting these to check for non-positive inputs and to throw a floating point exception?
I guess he expects (-2)**(-2) = 1/4.
But defined this way, (**) would be rather discontinuous._______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.