Change default float format

[mperham]

The current format is lossy in the extreme. This prints the Float as-is with 8 digits of precision.

float = Time.now.epoch_f
p float.to_json

str = String.new(22) do |buffer|
  LibC.snprintf(buffer, 22, "%.8f", float)
  len = LibC.strlen(buffer)
  {len, len}
end
p str

$ crystal tmp.cr
"1.46413e+09"
"1464131025.34958601"

Does the size 22 constant need to be adjusted?

[jhass]

I wonder if we still need the LibC.snprintf call actually, given we got sprintf / String#% these days. Well, at the end of the day it calls LibC.snprintf too. Still, might be better to have only one call to it for when we replace it with a Crystal implementation?

Anyway, specs would be awesome :)

[mperham]

I don't have a Crystal development environment set up just yet so I can't submit any non-trivial patches. Closing this since it's half-baked.

[jhass]

Oh that's easy if you already have Crystal installed it should be matter of running make in the repo and using bin/crystal to run the compiler & standard library of the repo. make spec runs the full testsuite, bin/crystal spec/... a specific file. :)

[asterite]

@mperham I wouldn't change the default Float#to_s. With your change:

puts 1.0 # => 1.00000000

Current behaviour:

# maybe it should be 1.0
puts 1.0 # => 1

There's a similar issue here (I don't know why it's closed): #2220 . Formatting floats is hard.