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Comment by waldrews | Hacker News Reader

In what language is 0.1 + 0.2 == 0.3? (I guess C# with decimal literals, but you'd have to specify that with a suffix)

I was curious and fired up sbcl:

    * (= (+ 0.1 0.2) 0.3)
    T
    * (type-of 0.1)
    SINGLE-FLOAT
    * (= 0.3d0 (+ 0.1d0 0.2d0))
    NIL
    * (type-of 0.1d0)
    DOUBLE-FLOAT

  Prelude> (0.1 + 0.2) == 0.3
  False
  Prelude Ratio> 1 % 10 + 2 % 10 == 3 % 10
  True

Or:

    ghci> 0.1 + 0.2 == (0.3 :: Ratio Int)
    True

Same type, less syntax.

That's cheating. Haskell isn't a language, it's The Word of Simon.

The question of which Simon is left as an exercise to the reader.

In Ruby:

0.1 + 0.2 == 0.3 => false

According to "The Ruby Programming Language", all languages that use IEEE-754 floating-point numbers have this problem.

In Ruby you can get around this by using BigDecimal.

>> require 'bigdecimal'

=> true

>> require 'bigdecimal/util'

=> true

>> 0.1.to_d + 0.2.to_d == 0.3.to_d

=> true

In Python

  >>> decimal.Decimal(".2") + decimal.Decimal(".1") == decimal.Decimal(".3")
  True
  >>> .2 + .1 == .3
  False