We can see this effect in practice in any language that supports floating point, such as Haskell: > truncate (16777216 - 1 :: Float) 16777215 > truncate (16777216 + 1 :: Float) 16777216 Subtracting 1 gives us the decremented number, but adding 1 had no effect with floating point math! * modules, I'd agree with you, those should be used as a last resort. This webpage is a HTML version of most of Bernie Pope's paper A Tour of the Haskell Prelude. main = print . The Float type is a single-precision floating point number. round x returns the nearest integer to x, the even integer if x is equidistant between two integers. Haskell/Type basics II, Float' instance Floating Double -- Defined in 'GHC. Haskell has two main floating point types: Float and Double. Since each floating point number is implemented in a finite number of bits, there exist numbers that can't be represented completely accurately. And of course, ... round, truncate, and so on. This isn't a haskell problem as much as a floating point problem. A Tour of the Haskell Prelude (and a few other basic functions) Authors: Bernie Pope (original content), Arjan van IJzendoorn (HTML-isation and updates), Clem Baker-Finch (updated for Haskell 98 hierarchical libraries organisation). As to GHC. Values of the built-in type Float are floating-point numbers: Main> 10 / 2.4 4.16667. You can convert a floating-point number to an Int or Integer using truncate and round. n is an integral number with the same sign as x; and ; f is a fraction with the same type and sign as x, and with absolute value less than 1.; The default definitions of the ceiling, floor, truncate and round functions are in terms of properFraction. We’ve gone over some of the conversions between similar types. Declare integer y and initialize it with the value of floating point number x.Ignore non-integer digits of x. The Haskell Prelude contains predefined classes, types, and functions that are implicitly imported into every Haskell program. sumU . The function properFraction takes a real fractional number x and returns a pair (n,f) such that x = n+f, and: . Trac metadata Double. Make sure to truncate towards zero: a negative x must yield the closest greater integer (not lesser). Float. Instead of using the methods from RealFrac I could simply use double2Int but I consider this a work-around. To make searching easy I've included a list of functions below. instance Enum Float where succ x = x + 1 pred x = x-1 toEnum = int2Float fromEnum = fromInteger. You can convert an Integer to a floating-point number (Float or Double) using fromInteger. mapU (floor :: Double -> Int) $ enumFromToFracU 0 100000000 Runs in 1 minute, 10 seconds: $ time ./henning 5000000050000000 ./henning 70.25s … Conversion Mania. ... (Fractional a) => Floating a where ... truncate x yields the integer nearest x between 0 and x, inclusive. As far as I can judge, double2Int does the same like truncate. @chi, ceiling, floor, truncate and fromIntegral are mentioned in the answer, so not quite sure why you brought them up. In GHC-6.6.1 these examples end with a stack overflow, but if I shorten the list, the time relations remain the same. Because Haskell has more than one type of floating point numbers, this "more generic" The usual way to convert an Int to a Double is to use fromIntegral, which has the type (Integral a, Num b) => a -> b. So now, we *do* have a good rule for truncate, but floor, ceiling and round turn out to be awesomely slow. Idiom #80 Truncate floating point number to integer. You can also see this by calculating 0.1 + 0.2, which awkwardly returns 0.30000000000000004 instead of 0.3. But it’s difficult to keep track of all the different ways to convert between values. 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