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

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.