Float parsing fails the toml-test when using Nim devel

[kaushalmodi]

This is the root cause of the recent failure of parsetoml tests on Travis weekly cron: https://travis-ci.org/github/NimParsers/parsetoml/builds/773550319

See nim-lang/Nim#18182 (comment) .

[kaushalmodi]

@PMunch

The parseDecimalPart is creating an incorrect outcome on Nim devel.

Here's a minimal reproducible example using parsetoml:

import std/[unittest]
import parsetoml # nimble install parsetoml

test "TOML float":
  check $("some_float = 0.123".parseString()["some_float"].getFloat()) == "0.123"

I dived into the code and ended up in parseDecimalPart:

parsetoml/src/parsetoml.nim

Lines 271 to 299 in 3760867

	proc parseDecimalPart(state: var ParserState): float64 =
	var
	nextChar: char
	invPowerOfTen = 10.0
	firstPos = true
	wasUnderscore = false
	result = 0.0
	while true:
	wasUnderscore = nextChar == '_'
	nextChar = state.getNextChar()
	if nextChar == '_':
	if firstPos or wasUnderscore:
	raise(newTomlError(state,
	"underscore must be surrounded by digit"))
	continue
	if nextChar notin strutils.Digits:
	if wasUnderscore:
	raise(newTomlError(state,
	"underscore must be surrounded by digit"))
	state.pushBackChar(nextChar)
	if firstPos:
	raise newTomlError(state, "decimal part empty")
	break
	result = result + (int(nextChar) - int('0')).float / invPowerOfTen
	invPowerOfTen *= 10
	firstPos = false

Here's a minimal example that shows where things are going wrong on devel:

import math, sugar

var
  invPowerOfTen = 10.0
  decimalPart: float64

for ch in ['1', '2', '3']: # 0.123
  decimalPart += (int(ch) - int('0')).float / invPowerOfTen
  dump ch
  dump decimalPart
  invPowerOfTen *= 10

doAssert $decimalPart == "0.123"

Output:

ch = 1
decimalPart = 0.1
ch = 2
decimalPart = 0.12000000000000001
ch = 3
decimalPart = 0.12300000000000001
/home/kmodi/sandbox/nim/bug_reports/t18182.nim(13) t18182
/home/kmodi/usr_local/apps/7/nim/devel/lib/system/assertions.nim(39) failedAssertImpl
/home/kmodi/usr_local/apps/7/nim/devel/lib/system/assertions.nim(29) raiseAssert
/home/kmodi/usr_local/apps/7/nim/devel/lib/system/fatal.nim(53) sysFatal
Error: unhandled exception: /home/kmodi/sandbox/nim/bug_reports/t18182.nim(13, 10) `$decimalPart == "0.123"` 

When I run the same code on latest Nim stable on Playground: https://play.nim-lang.org/#ix=3oMQ, the output is:

ch = 1
decimalPart = 0.1
ch = 2
decimalPart = 0.12
ch = 3
decimalPart = 0.123

These is how far I could get. I don't understand the low level details on why the outcome is different. Also copying @timotheecour in case he's interested in helping fix this in parsetoml.

[kaushalmodi]

OK, so this fails on devel:

nim --eval:'assert 0.1 + 0.02 == 0.12'

I am still not sure if this is something that should be fixed in Nim devel. But assuming it's not, can something like this be done in parsetoml:

nim --eval:'assert (10 + 2)/100.0 == 0.12'

Above passes. So we first need to figure out how many decimal point digits we have, scale up the decimal parts and then normalize using a float division.

---

Or somehow strutils.parseFloat needs to be used as suggested in nim-lang/Nim#18182 (comment) so that it does the Right Thing.

[timotheecour]

OK, so this fails on devel:
nim --eval:'assert 0.1 + 0.02 == 0.12'

not just nim devel, 1.4.4 too, and indeed, 0.1 + 0.02 != 0.12 with FP semantics.

Or somehow strutils.parseFloat

parseutils.parseFloat is IMO what should be used