Add js BigInts

[juancarlospaco]

• Add arbitrary precision integers.
• Add bigints to standard library #14696 (comment)
• runnableExamples with doAssert, styleCheck, changelog, documentation with links.

[ringabout]

We really need bigInt(JS backend) in compiler
See:
nim-lang/RFCs#187
#13298

[juancarlospaco]

I know and the C/C++ too, still stuff gotta start somewhere... 🤷

[timotheecour]

excellent!

[ringabout]

LGTM after addressing some comments.

[timotheecour]

@Araq friendly ping on this, is there anything else needed? from my perspective every blocker was addressed, and the rest can be done in future PRs

[timotheecour]

@juancarlospaco what was the reason again for:

func `**`*(x, y: JsBigInt): JsBigInt {.importjs: "((#) $1 #)".}
# instead of
func `**`*(x, y: JsBigInt): JsBigInt {.importjs: "(# $1 #)".}

?
IIRC you mentioned the reason but I can't remember; the 2nd form seems to work at least here:

import std/jsbigints
proc main()=
  block:
    var a = big"2"
    var b = big"3"
    var c = a ** b
    echo c
  block:
    var c = [big"2"]
    echo c[0] ** c[0]
main()

this is also useful for the description of P3 in nim-lang/RFCs#315

[juancarlospaco]

The operator is introvert and shy, and likes to be surrounded by parenthesis.

#16409 (comment)

[timotheecour]

thanks, this helped.

here's an even clearer explanation: the problem is when there's a unary operator
(EDIT)

func `**`*(x, y: int): int {.importjs: "((#) ** #)".}
func `***`*(x, y: int): auto = (x, y)
func `****`*(x, y: int): int {.importjs: "(# ** #)".}

proc id(a: auto): auto = a

doAssert id(-2 *** 3) == (-2, 3) # checks that - has higher precedence than ***
doAssert id(-2 ** 3) == -8 # ok
doAssert id(`****`(2, 3)) == 8
doAssert id(`****`(1+1, 3)) == 8

doAssert id(-2 **** 3) == -8 #  # js error: SyntaxError: Unary operator used immediately before exponentiation expression. Parenthesis must be used to disambiguate operator precedence
# doAssert id((-2) **** 3) == -8 # ditto
# doAssert id(`****`((-2), 3)) == -8 # ditto

• TODO: add regression test to https://github.com/xflywind/Nim/blob/8f9aac66f459d5031ccfa33562617a4b2900a035/tests/js/tjsffi.nim#L362, the existing one is not enough, as demonstrated here

note

for func ***(x, y: JsBigInt): JsBigInt {.importjs: "((#) $1 #)".}, the (#) is also needed but it's tricker to see why:

# suppose someone adds this other proc that doesn't have ():
func neg(this: JsBigInt): JsBigInt {.importjs: "-#".}
let b = big"1"
echo (-b ** big"3") # works
echo (b.neg ** big"3") # js error

[timotheecour]

@juancarlospaco 2nd question: you're using outer parens in all your importjs procs (eg: importjs: "(# + #)". Is that actually required, and can you think of an example where ommitting those would result in incorrect code? I couldn't find such example yet

[timotheecour]

@juancarlospaco also, followup PR welcome for missing procs (eg: bitwise and, or) as mentioned by @xflywind

[ringabout]

Ah, I already make that PR :)
#16843