Let

A simple demo language that builds upon FE.

Usage

USAGE:
  let [-?|-h|--help] [-v|--version] [-d|--dump] [-e|--eval] [<file>]

Display usage information.

OPTIONS, ARGUMENTS:
  -?, -h, --help
  -v, --version           Display version info and exit.
  -d, --dump              Dumps the let program again.
  -e, --eval              Evaluate the let program.
  <file>                  Input file.

Use "-" as <file> to output to stdout.

Building

If you have a GitHub account setup with SSH, just do this:

git clone --recurse-submodules git@github.com:leissa/let.git

Otherwise, clone via HTTPS:

git clone --recurse-submodules https://github.com/leissa/let.git

Then, build with:

cd let
cmake -S . -B build -DCMAKE_BUILD_TYPE=Debug
cmake --build build -j $(nproc)

For a Release build simply use -DCMAKE_BUILD_TYPE=Release.

Invoke the interpreter like so:

./build/bin/let test/test.let -e

Grammar

p = s ... s EOF         (* program *)
  ;

s = ';'                 (* empty statement *)
  | 'let' ID '=' e ';'  (* let statement *)
  | 'print' e ';'       (* print statement *)
  ;

e = LIT                 (* literal expression *)
  | ID                  (* identifier expression *)
  | '(' e ')'           (* parenthesized expression *)
  | OP1 e               (* unary expression *)
  | e OP2 e             (* binary expression *)
  ;

where

• LIT = [0-9]+
• ID = [a-zA-Z][a-zA-Z0-9]*
• OP1 is one of: +, -
• OP2 is one of: *, +, -, /

In addition, Let supports

• /* C-style */ and
• // C++-sytle comments.

For the sake of the demo, Let is case-insensitive.

Precedence

Ambiguities in the expression productions are resolved according to the operator precedence that is summarized in the following table (strongest binding first):

Operator	Description
+e, -e	unary plus, unary minus
*, /	multiplication, division
+, -	addition, subtraction

All binary operators are left associative.

Semantics

All calculations use 64-bit unsigned integer wrap-around arithmetic. Division by zero yields zero.