Identifier equality

Two identifiers are considered equal if the following algorithm returns true:

proc sameIdentifier(a, b: string): bool =
  a[0] == b[0] and
    a.replace("_", "").toLowerAscii == b.replace("_", "").toLowerAscii

That means only the first letters are compared in a case-sensitive manner. Other letters are compared case-insensitively within the ASCII range and underscores are ignored.

This rather unorthodox way to do identifier comparisons is called partial case-insensitivity and has some advantages over the conventional case sensitivity:

It allows programmers to mostly use their own preferred spelling style, be it humpStyle or snake_style, and libraries written by different programmers cannot use incompatible conventions. A Nim-aware editor or IDE can show the identifiers as preferred. Another advantage is that it frees the programmer from remembering the exact spelling of an identifier. The exception with respect to the first letter allows common code like var foo: Foo to be parsed unambiguously.

Note that this rule also applies to keywords, meaning that notin is the same as notIn and not_in (all-lowercase version (notin, isnot) is the preferred way of writing keywords).

Historically, Nim was a fully style-insensitive language. This meant that it was not case-sensitive and underscores were ignored and there was not even a distinction between foo and Foo.