floating point - Haskell: Unexpected output for expression [0, 0.1 .. 1]

Question

When evaluating the expression:

*main> [0, 0.1 .. 1]

I was actually expecting:

 [0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1]

But I was quite shocked to see the output be

[0.0,0.1,0.2,0.30000000000000004,0.4000000000000001,0.5000000000000001,0.6000000000000001,0.7000000000000001,0.8,0.9,1.0]

Why does Haskell produce that result upon evaluation?

Answer 1

This is a result of the imprecision of floating point values, it isn't particular to Haskell. If you can't deal with the approximation inherent in floating point then you can use Rational at a high performance cost:

> import Data.Ratio
Data.Ratio> [0,1%10.. 1%1]
[0 % 1,1 % 10,1 % 5,3 % 10,2 % 5,1 % 2,3 % 5,7 % 10,4 % 5,9 % 10,1 % 1]

Just to hammer the point home, here's Python:

>>> 0.3
0.29999999999999999

And here's C:

void main() { printf("%0.17f
",0.3); }

$ gcc t.c 2>/dev/null ; ./a.out
0.29999999999999999