expression 'a' is of type 'int' and has to be used (or discarded) - nim-lang

Question

I'm trying to rewrite the cryptomath code from here and I'm getting this when I try to compile to JS;
Hint: used config file '/home/*******/.choosenim/toolchains/nim-1.6.8/config/nim.cfg' [Conf]
Hint: used config file '/home/*******/.choosenim/toolchains/nim-1.6.8/config/config.nims' [Conf]
...........................................................
/home/*******/nim/cryptomath.nim(6, 9) Error: expression 'a' is of type 'int' and has to be used (or discarded)
This is my code:
import std/random
import std/math
randomize()
proc gcd*(a: int, b: int): int =
while a != 0:
a, b = floorMod(b,a), a # fails here apparently
return (b+a)-a
proc find_mod_inverse*(a: int, m: int): int =
if gcd(a,m) != 1:
return -1
var
u1 = 1
u2 = 0
u3 = a
v1 = 0
v2 = 1
v3 = m
q = -1
while v3 != 0:
q = floorDiv(u3,v3)
v1 = (u1 - q * v1)
v2 = (u2 - q * v2)
v3 = (u3 - q * v3)
u1 = v1
u2 = v2
u3 = v3
return floorMod(u1,m)
I tried adding this, but it did nothing
discard a
before the end of the function

Answer 1

Your code has two issues:
Function arguments in Nim are immutable by default, so if you want to overwrite them locally, you need to shadow them or use var
Nim syntax for multiple variable assignment is different from Python and is done with tuple-like syntax
Fixed code would look like this:
import std/random
import std/math
randomize()
proc gcd*(a: int, b: int): int =
var (a, b) = (a, b)
while a != 0:
(a, b) = (floorMod(b,a), a)
return (b+a)-a
proc find_mod_inverse*(a: int, m: int): int =
if gcd(a,m) != 1:
return -1
var
u1 = 1
u2 = 0
u3 = a
v1 = 0
v2 = 1
v3 = m
q = -1
while v3 != 0:
q = floorDiv(u3,v3)
v1 = (u1 - q * v1)
v2 = (u2 - q * v2)
v3 = (u3 - q * v3)
u1 = v1
u2 = v2
u3 = v3
return floorMod(u1,m)
The compiler error is unclear, I agree.
Also, just a tip - keep in mind that standard Nim integers are limited by the architecture's native integer size, so if you want to operate on big numbers, you need to use a separate library.