commit	231861d38d139e5a9fbb27fcda3d6a74a5c1b65d	[log] [tgz]
author	Sketch🕴️ <skallywag@sketch.dev>	Sat Feb 28 21:12:18 2026 +0400
committer	Sketch🕴️ <skallywag@sketch.dev>	Sat Feb 28 21:12:18 2026 +0400
tree	7345ee44716147a46b39109907161d646afef673
parent	519412600929d0dc64638699a9fd497a4f92416a [diff]

tree: 7345ee44716147a46b39109907161d646afef673

README.md

matheval

A math expression evaluator with user-defined functions and an interactive REPL, written in Go.

Features

Arithmetic operators: +, -, *, /
Parentheses for grouping
Floating point numbers (including .5 syntax)
Correct operator precedence (* and / bind tighter than + and -)
Left-to-right associativity
User-defined functions with f(x) = x + 1 syntax
Multi-parameter functions: f(x, y) = x + y
Functions can call other user-defined functions (late binding)
Clear error messages with position reporting

Build

go build -o matheval ./cmd/matheval

Usage

Run the REPL:

./matheval

Then type expressions and define functions:

>> 2 + 3 * 4
14
>> (2 + 3) * 4
20
>> 7 / 2
3.5
>> 1 / 0
error: division by zero
>> square(x) = x * x
defined square
>> square(5)
25
>> add(x, y) = x + y
defined add
>> add(2, 3)
5
>> hyp(a, b) = square(a) + square(b)
defined hyp
>> hyp(3, 4)
25

Press Ctrl+D (EOF) to exit.

You can also pipe input:

echo "2 + 3" | ./matheval

Architecture

Input string → Lexer → Parser → AST → Evaluator → Result

Package	Responsibility
`token`	Token types and data structures
`lexer`	Tokenizes input string
`ast`	AST node types (`NumberLit`, `BinaryExpr`, `Ident`, `FuncCall`, `FuncDef`)
`parser`	Recursive-descent parser (expressions and function definitions)
`evaluator`	Stateful evaluator with function registry
`repl`	Read-eval-print loop with persistent state

Grammar

line     → funcdef | expr
funcdef  → IDENT '(' params ')' '=' expr
params   → IDENT (',' IDENT)*
expr     → term (('+' | '-') term)*
term     → factor (('*' | '/') factor)*
factor   → NUMBER | IDENT '(' args ')' | IDENT | '(' expr ')'
args     → expr (',' expr)*

Function Rules

Define: name(params) = body — body is any expression using the parameters
Call: name(args) — arguments are arbitrary expressions
Redefinition: Not allowed (returns an error)
Cross-calling: Functions can call other user-defined functions
Late binding: References resolved at call time, not definition time

Tests

go test ./...

This runs unit tests for each package plus integration tests covering the full pipeline.