RFC: Revise `char` primitive

State of char type

ReScript has the char primitive type, which is rarely used. (I was one of those who used char to handle ASCII keycodes)

https://rescript-lang.org/docs/manual/latest/primitive-types#char

Note: Char doesn't support Unicode or UTF-8 and is therefore not recommended.

The char doesn't support Unicode, but only supports UTF-16 codepoint.

let a = '👋'

compiles to

let a = 128075;

Its value is the same as '👋'.codePointAt(0) result in JavaScript, which means that in the value representation, char is equivalent to int (16-bit subset).

Then, why don't we use just int instead of char?

1. char literals are automatically compiled to codepoints. This is much more efficient than string representation when dealing with the Unicode data table.
2. char supports range pattern (e.g. 'a' .. 'z') in pattern matching. This is very useful when writing parsers.

However, a char literal is not really useful to represent a Unicode character because it doesn't cover the entire Unicode sequence. It only returns the first codepoint value and discards the rest of the character segment.

To avoid problems, we should limit the value range of char literal to the BMP(Basic Multilingual Plane, U+0000~U+FFFF).

Suggestion

I suggest some changes that would keep the useful parts of char but remove its confusion.

• Get rid of char type or make it an alias of int
• Keep char literal syntax, but with internal representation as regular integers
• Limit the char literal range to BMP in the syntax level.
• Support range patterns for regular integers
• Remove the Char module.

Start with the linked primitive-types#char documentation and review the current Char module and compiler handling of char literals and range patterns. Clarify the representation, BMP limits, integer range patterns, and whether Char is removed; done requires an agreed design and corresponding implementation and tests, none of which are named here.