Am Donnerstag, 8. Januar 2009 17:45 schrieb Sergei Winitzki:
Subject: how to print a floating-point number? hi,
I am very new to Haskell. I am trying to benchmark the CPU time needed for a computation, and I can't figure out how to print a floating-point number.
My code is as follows, and I expected it to work:
import System.CPUTime main = do let result = some_computation print result time <- getCPUTime -- this is an Integer that needs to be divided by 1e12 to get time in seconds print (time / 1.0e12) -- I want this to print a floating-point number
You have to convert the Integer to a floating point number first, use fromInteger or fromIntegral for that. Haskell does not do automatic conversion between numeric types.
But this does not compile. Error message: No instance for (Fractional Integer) arising from use of `/' at fact1.hs:18:15-28 Possible fix: add an instance declaration for (Fractional Integer) In the first argument of `print', namely `(time1 / 1.0e12)'
I thought this should work because e.g. 12 / 7 evaluates to 1.7142857142857142 in ghci.
That is because numeric *literals* are polymorphic, as they are parsed as e.g. "fromInteger 12" if it's an integer literal or "fromRational 3.24" if it's a non-integer literal.
I understand this is some problem with types, but surely it is fixed not by adding any instance declarations but perhaps by using some Prelude or Numeric function. But which one? I tried everything I could find in the documentation: showFloat, adding ::Float everywhere, adding fromIntegral, etc.etc. - nothing works. All the books and the tutorials I looked at seem to discuss at length such nice things as Fibonacci numbers and recursive factorial functions rather than a practical problem like this.
When you are looking for a function, it's a good idea to ask hoogle (http://haskell.org/hoogle/) for a function of appropriate type. Asking for a function Integer -> Double, the abovementioned are results 1 and 3.
help will be much appreciated!
Sergei
HTH, Daniel