Conversion between numerical types in Haskell must be done explicitly. This is unlike many traditional languages (such as C or Java) that automatically coerce between numerical types.
1 Converting from and between integral types (integer-like types)
- , which are arbitrary-precision integers, often called "bignum" or "big-integers" in other languages, andInteger
- , which fixed-width machine-specific integers with a minimum guaranteed range ofInt
229 − 1. In practice, its range can be much larger: on the x86-64 version of Glasgow Haskell Compiler, it can store any signed 64-bit integer.
fromIntegral :: (Num b, Integral a) => a -> b
fromInteger :: Num a => Integer -> a
toInteger:: Integral a => a -> Integer
2 Converting from real and between real-fractional types (rational-like types)
- , which are arbitrary-precision fractions, andRational
- , which are double-precision floating-point numbers.Double
realToFrac:: (Real a, Fractional b) => a -> b
It can also be used to convert between real-fractional types.There are special cases for converting from
fromRational :: Fractional a => Rational -> a
toRational :: Real a => a -> Rational
3 Converting from real-fractional numbers to integral numbers
This is an inherently lossy transformation since integral types cannot express non-whole numbers. Depending on how you wish to convert, you may choose any of the following:
ceiling :: (RealFrac a, Integral b) => a -> b floor :: (RealFrac a, Integral b) => a -> b truncate :: (RealFrac a, Integral b) => a -> b round :: (RealFrac a, Integral b) => a -> b
4 Converting between different floating-point precisionsConversion between
float2Double :: Float -> Double double2Float :: Double -> Float
5 Automatic conversion
Repeatedly people ask for automatic conversion between numbers. This is usually not a good idea; for more information, refer to the thoughts about a Generic number type.
Hi, I am trying to write some functions that convert between two coordinate systems. The first coordinate system, which ill call coord1, starts in the upper left at (0, 0) and ends in the lower right at (500, 500). Coordinates in coord1 have type (Int, Int). The second coord system, which I'll call coord2, starts in the lower left at (0.0, 0.0) and ends in the upper right at (1.0, 1.0). Coords in coord2 have type (Float, Float). I was hoping someone could help me figure out how I can rewrite the two functions below so that the type checker will accept them.
coord1ToCoord2 :: (Int, Int) -> (Float, Float) coord1ToCoord2 (x, y) = (x/500, (500-y)/500) coord2ToCoord1 :: (Float, Float) -> (Int, Int) coord2ToCoord1 (x, y) = (500/(1/x), 500 - 500/(1/y))
examples of what i want. i think i have the logic right :)
coord1ToCoord2 (0, 0) -> (0.0, 1.0) coord1ToCoord2 (250, 250) -> (0.5, 0.5) coord1ToCoord2 (350, 350) -> (0.7, 0.3) coord1ToCoord2 (500, 500) -> (1.0, 0.0) coord2ToCoord1 (0.0, 0.0) -> (0, 500) coord2ToCoord1 (0.5, 0.5) -> (250, 250) coord2ToCoord1 (0.7, 0.7) -> (350, 150) coord2ToCoord1 (1.0, 1.0) -> (500, 0)
One of the thing that confused me was that I expected 500 to be an Int, but in fact the literals are automatically converted to a correct Num instance.
The solution here was to use fromIntegral and round :
coord1ToCoord2 :: (Int, Int) -> (Float, Float) coord1ToCoord2 (x, y) = (fromIntegral x/500, (500 - fromIntegral y)/500) coord2ToCoord1 :: (Float, Float) -> (Int, Int) coord2ToCoord1 (x, y) = (round (500 * x), round (500 - 500 * y))