[Belt] Stricter parse for Int and Float

[rcherrueau]

The actual implementation of Belt.Int.fromString relies on the
Javascript parseInt function [0]. That function parses Strings in a
permissive manner. For instance, if the function encounters an
alphabetical character, it returns the value up to that character:

parseInt("123a456") // output: 123

The same applies for Belt.Float.fromString that relies on
parseFloat [1] and have a similar interpretation.

This commit proposes a stricter implementation of Int and Float parsing
using the Javascript Number constructor [2]. The Belt.Int.fromString
uses the more specific Math.trunc function [3], that parses string
similarly to Number and then returns the Int part of that number.

Relying only on the Number constructor keeps the implementation simple.
However, there exists one last quirk while using it with an empty
string. It returns the 0 value:

Number("") // ouptput: 0

Consequently, Belt.Int.fromString("") will return 0. Tests for
Belt.(Int/Float).fromString have been updated in order to reflect that.

Fix #3732

[0] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/parseInt
[1] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/parseFloat
[2] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/Number
[3] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/trunc

[bobzhang]

relevant #3732, I think it is okay to introduce a breaking change in v10.
Number seems not safe:

Number("90071992547409922")
90071992547409920

I would suggest here we just use a handwritten one which has clear specification, reference implementation (need some error/overflow checking):

function codePointInt(value) {
  var result = 0;
  var length = value.length;
  for (var i=0; i < length; i++) {
    var code = value.codePointAt(i);
    result = 10 * result + (code - 48)  | 0
  }
  return result;
}

[rcherrueau]

I understand.

In that case, may I suggest to follow an implementation similar to Elm [0]. I tested its performance against the implementations in #3732 (comment), and the result is really good [1]. However, even with that implementation the parsing of 90071992547409922 fails, because that number is higher than Number.MAX_SAFE_INTEGER and thus should be represented as a BigInt [2]. I don't know how this should be handle in rescript.

References:

[0] https://github.com/elm/core/blob/65cea00afa0de03d7dda0487d964a305fc3d58e3/src/Elm/Kernel/String.js#L278-L297

[1] https://rebench.github.io/?rebench-data=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-QwSJPzoDQMBKUQCFMUjyJ6HwxCwXhdl8DB+HwXACEYZNqPIkgaH4MRNXkBQXnkhsvF4EThicLBLCM4DeHAIZEj01iXkQTDzEwl5Krkktt1M5TzJYDEsRxbc8QMoy+oU78lK-CrBms1VjEovKCtkIqQOc8C3LtDyYNdVwfNAQY4DoAKZF8EReHUjAAHcaHMLAEsUCjdmjDQADE1sK16nB0RwAD5t0QO6Y3CNiowkfgSAkHRNp0XrZ0aEMNNoIGcPyqhNFRzS9G3RoqAKLgqCwGi2M+8xEs-bGJDERAFF6hxAYwTDBCoVUJhYKNvvy37iqcsDXOtPboOdQ6PXdd0gA

[2] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt

[cristianoc]

I think it is okay to introduce a breaking change in v10.

Since v10 is coming soon, perhaps this can be revisited.

[cristianoc]

Feel free to reopen if still relevant.