Re: [Haskell-beginners] truncate results depend on strict/lazy

On Tue, Sep 10, 2013 at 05:26:08AM +0700, Kim-Ee Yeoh wrote:

On Tue, Sep 10, 2013 at 5:08 AM, Bryan Vicknair wrote:

...

And all of a sudden, the parsing code doesn't work again!:

Prelude Data.Text Lib> validateVal $ pack "0.12" Success (Just 11)

This might be due to a floating-point roundoff error since 0.12 doesn't have a finite binary representation.

The function truncate does exactly what it says, so truncate (100 * 0.1199999) evaluates to 11.

Are you sure you don't want "round" instead of "truncate"?

-- Kim-Ee

My first thought, as always with Floats, is that there was a binary representation problem. If it were just that I wouldn't mind, but it is the inconsistent evaluation of the following expression that is really bothering me:

truncate ((0.12::Float) * (100::Float))

let f = 0.12::Float truncate (f * 100)

In GHCI, I get 12: truncate ((0.12::Float) * (100::Float)) But in my web app and in this project it gives me 11: https://bitbucket.org/bryanvick/truncate/overview Whatever the behavior of truncate is, given the same input it should give the same output. In the project referenced above, the input to validateVal and unsafeValidateVal is always "0.12", but the output is 11 for the first and 12 for the second. The only difference between the two functions is that the unsafe version performs IO.