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Comment by edflsafoiewq | Hacker News Reader

BigDecimal("123.45") != 123.45.to_d. You should always prefer the former. In the latter, 123.45 is first read as a float, which introduces an error because it is not exactly representable as a float.

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Ruby 2.7.2, just fire up irb:

BigDecimal("123.45") == 123.45.to_d

=> true

Shows you what I know. I thought I had tested this. I guess it keeps only a certain amount of precision.

That is just a coincidence (decimal ending in "5"), plus the fact that `.to_d` does some cleanup preprocessing.

Counterexample:

  BigDecimal("1.5999999999999996") == 1.5999999999999996.to_d
  => false

The most ergonomic way to initialize decimal numbers is IMO

  "123.45".to_d

(Note the quotes around the number. `.to_d` is being called on a string.)

BigDecimal's only lossless API is through strings. It would be nice to have native support for decimals at the parser level:

  0d123.45 => BigDecimal("123.45")

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