nim lang - Why same strings are not equal in Nim?

Question

The string generated by the JSON package is not equal to the same looking string, why?

The last equality check in code below is false, why? playground

import strformat, json

type Price = tuple[price: float, currency: string]

func `%`*(v: Price): JsonNode = %(fmt"{v.price} {v.currency}")

let v: Price = (214.05, "USD")
let s: string = (%(v)).pretty

echo s                  # => "214.05 USD"
echo s == "214.05 USD"  # => false, why?

Answer 1

The echo proc can sometimes be deceptive in terms of representing strings, so I prefer to throw in the repr proc which makes sure to represent strings (and other objects) in a more clear way. See the output I get when I add the following lines to the example, representing both sides of the comparison:

echo repr(s)
echo repr("214.05 USD")
==>
0x105f5c0b8""214.05 USD""
0x105f1cf70"214.05 USD"

Through this representation we can see the strings are really not equal. Your % proc formats the contents of the Price tuple as a space separated string node, and the JSON pretty proc converts that string node to its JSON representation, which includes double quotes. Thus, to make the last comparison equal we need to write it like this:

echo s == ""214.05 USD""

Another possibility would be to write the comparison accessing the JsonNode.str variant field, which avoids involving the mischievous pretty:

echo (%(v)).str == "214.05 USD"