On Fri, Mar 27, 2009 at 09:12:46AM +0000, 7stud wrote:
Daniel Fischer
writes: (/) in the first equation is at type Double -> Double -> Double.
Then why don't I get an error here:
Prelude> 2 / 4 0.5
There are several things going on here. The first is that numeric literals are overloaded; integral numeric literals (like 2) can be of any numeric type. What actually happens is that they get wrapped in a call to fromIntegral. The other thing going on is ghci's type defaulting: since it doesn't otherwise know what type this expression should be, it picks Double. (It would also pick Integer if that worked, but Integer doesn't work here because of the (/).)
Prelude> 2 / fromIntegral 4 0.5
From the above discussion you can see that this is exactly the same as the first expression.
And why does this happen:
Prelude> let x = 2 Prelude> :type x x :: Integer
This is because of the defaulting again. Since there are no constraints on the type of x this time, it picks Integer.
Prelude> x / fromIntegral 4
<interactive>:1:0: No instance for (Fractional Integer) arising from a use of `/' at <interactive>:1:0-17 Possible fix: add an instance declaration for (Fractional Integer) In the expression: x / fromIntegral 4 In the definition of `it': it = x / fromIntegral 4
Of course, now that x has type Integer, this is not well-typed; you can't divide Integers.
And how do I read this type:
Prelude> :type fromIntegral fromIntegral :: (Num b, Integral a) => a -> b
What does the => mean?
The => indicates class constraints. You can read this as "fromIntegral has type a -> b, as long as b is an instance of the Num class, and a is an instance of the Integral class." That is, fromIntegral can convert a value of any Integral type (Int, Integer) to a value of any Num type. -Brent