Meow Programming Language Compiler Internals

This document describes the internal architecture of the Meow Programming Language compiler for contributors who want to understand or modify the compilation pipeline.

Pipeline Overview

flowchart TD
    src[".nyan source"]
    lexer["Lexer<br/>pkg/lexer"]
    parser["Parser<br/>pkg/parser"]
    checker["Checker<br/>pkg/checker"]
    codegen["Codegen<br/>pkg/codegen"]
    gobuild["go build"]
    bin(["binary"])

    src --> lexer
    lexer -- "iter.Seq[Token]" --> parser
    parser -- "AST" --> checker
    checker -- "TypeInfo" --> codegen
    codegen -- "Go source" --> gobuild
    gobuild --> bin

The pipeline is orchestrated by compiler/compiler.go:

1. Lexer tokenizes .nyan source into a stream of tokens
2. Parser builds an AST from the token stream
3. Checker performs type checking and collects type information
4. Codegen transforms the AST into Go source code
5. go build compiles the Go source to a native binary

Lexer (pkg/lexer/)

Design

The lexer produces an iter.Seq[Token] — a Go 1.26 push-based iterator. This means the lexer doesn’t allocate a slice of all tokens upfront; instead, it yields tokens lazily as they’re consumed.

Token Emission

func Lex(source, filename string) iter.Seq[token.Token] {
    return func(yield func(token.Token) bool) {
        // scan characters, yield tokens
    }
}

Scanning

The lexer operates character-by-character:

1. Skips whitespace (spaces, tabs, carriage returns)
2. Recognizes single/multi-character operators (==, !=, |=|, ~>, .., =>)
3. Scans identifiers and looks them up in the keyword table (token.LookupIdent)
4. Scans numeric literals (integers and floats)
5. Scans string literals (double-quoted, with escape sequences)
6. Handles line comments (#) and block comments (-~ ... ~-)
7. Emits NEWLINE tokens as statement separators

Position Tracking

Every token carries a Position with file name, 1-based line number, and column number.

Parser (pkg/parser/)

Design

The parser uses Pratt parsing (top-down operator precedence) for expressions, with recursive descent for statements. The token stream arrives as iter.Seq[Token], which is converted to a pull-based iterator via iter.Pull:

func New(tokens iter.Seq[token.Token]) *Parser {
    next, stop := iter.Pull(tokens)
    p := &Parser{next: next, stop: stop}
    p.advance()
    p.advance()
    return p
}

The parser maintains two tokens: cur (current) and peek (lookahead).

Precedence Levels

const (
    precNone  = iota
    precCatch // ~>
    precOr    // ||
    precAnd   // &&
    precEq    // == !=
    precCmp   // < > <= >=
    precPipe  // |=|
    precAdd   // + -
    precMul   // * / %
    precUnary // ! -
    precCall  // () [] .
)

Expression Parsing

The core of Pratt parsing:

func (p *Parser) parseExpr(minPrec int) ast.Expr {
    left := p.parsePrefix()      // Parse prefix (literal, ident, unary, etc.)
    for {
        prec := p.infixPrec(p.cur.Type)
        if prec <= minPrec {
            break
        }
        left = p.parseInfix(left, prec)  // Parse infix (binary, pipe, catch)
    }
    return left
}

Prefix parsers handle: literals, identifiers, unary operators, lambdas, lists, maps, match expressions, and grouped expressions (...).

Infix parsers handle: binary operators, pipe |=|, and catch ~>.

Statement Parsing

parseStmt() dispatches on the current token type:

Newline Handling

Newlines are significant as statement terminators. The parser skips consecutive newlines and comments between statements via skipNewlines(). Within an expression, newlines within brackets [...], braces {...}, and parentheses (...) are ignored.

AST (pkg/ast/)

Node Hierarchy

classDiagram
    class Node {
        <<interface>>
    }
    class Expr {
        <<interface>>
        produces a value
    }
    class Stmt {
        <<interface>>
        performs an action
    }
    class Pattern {
        <<interface>>
        for pattern matching
    }
    class TypeExpr {
        <<interface>>
        type annotations
    }

    Node <|-- Expr
    Node <|-- Stmt
    Node <|-- Pattern
    Node <|-- TypeExpr

    Expr <|-- IntLit
    Expr <|-- FloatLit
    Expr <|-- StringLit
    Expr <|-- BoolLit
    Expr <|-- NilLit
    Expr <|-- Ident
    Expr <|-- UnaryExpr
    Expr <|-- BinaryExpr
    Expr <|-- CallExpr
    Expr <|-- LambdaExpr
    Expr <|-- ListLit
    Expr <|-- MapLit
    Expr <|-- IndexExpr
    Expr <|-- PipeExpr
    Expr <|-- CatchExpr
    Expr <|-- MatchExpr
    Expr <|-- MemberExpr

    Stmt <|-- VarStmt
    Stmt <|-- FuncStmt
    Stmt <|-- ReturnStmt
    Stmt <|-- IfStmt
    Stmt <|-- RangeStmt
    Stmt <|-- FetchStmt
    Stmt <|-- KittyStmt
    Stmt <|-- ExprStmt

    Pattern <|-- LiteralPattern
    Pattern <|-- RangePattern
    Pattern <|-- WildcardPattern

    TypeExpr <|-- BasicType

Key Nodes

• PipeExpr: Left |=| Right — desugared to a function call in codegen
• CatchExpr: Left ~> Right — desugared to GagOr in codegen
• RangeStmt: Supports both count form (Start=nil) and range form (Start!=nil, Inclusive=true)
• KittyStmt: Defines struct types; collected before code generation so constructors can be generated

Type Checker (pkg/checker/)

Two-Pass Design

The checker performs two passes over the AST:

1. Declaration registration: Scans all function declarations and kitty definitions, recording their type signatures in FuncTypes
2. Type checking: Walks the AST, verifying type annotations, checking function calls, and recording expression types in ExprTypes

TypeInfo

The checker produces a TypeInfo struct passed to codegen:

type TypeInfo struct {
    FuncTypes map[string]types.FuncType    // function name → type signature
    ExprTypes map[ast.Expr]types.Type      // expression → inferred type
    VarTypes  map[string]types.Type        // variable name → declared type
}

Gradual Typing

The type system is gradual — untyped code coexists with typed code. The AnyType represents dynamically-typed values. Functions are considered “fully typed” only when all parameters and the return type have concrete types.

Scope Stack

Variables are tracked in a scope stack. Function bodies push a new scope containing the parameters. The checker resolves variable references by walking up the scope chain.

Codegen (pkg/codegen/)

Value Boxing

In untyped mode, all values are boxed as meow.Value:

Typed Code Generation

When a function is “fully typed” (all params and return have concrete types), codegen generates native Go types:

The typed path avoids boxing overhead:

meow add(a int, b int) int { bring a + b }

Generates:

func add(a int64, b int64) int64 {
    return (a + b)
}

When typed functions are called from untyped contexts, values are unboxed at call sites and re-boxed for the return value.

Stdlib Import Resolution

The stdPackages map defines available packages:

var stdPackages = map[string]string{
    "file":    "github.com/135yshr/meow/runtime/file",
    "http":    "github.com/135yshr/meow/runtime/http",
    "testing": "github.com/135yshr/meow/runtime/testing",
}

nab "file" registers the import, and member calls like file.snoop(x) are generated as meow_file.Snoop(x) — the function name is capitalized by capitalizeFirst.

Pipe Desugaring

The pipe |=| is desugared to a function call:

x |=| f(y)    →    f(x, y)
x |=| f       →    f(x)

Catch Desugaring

The catch ~> is desugared to GagOr:

expr ~> fallback

Becomes:

meow.GagOr(meow.NewFunc("~>", func(args ...meow.Value) meow.Value {
    return <expr>
}), <fallback>)

Kitty (Struct) Handling

Kitty definitions are collected in a pre-pass (collectKittyDefs). They don’t generate Go struct types — instead, they use the runtime Kitty value with dynamic field lookup:

Cat("Nyantyu", 3)

Generates:

meow.NewKitty("Cat", []string{"name", "age"}, meow.NewString("Nyantyu"), meow.NewInt(3))

Field access cat.name generates cat.(*meow.Kitty).GetField("name").

Test Mode

In test mode (GenerateTest), the codegen:

1. Auto-imports the testing package
2. Collects test_ prefixed functions and wraps them with meow_testing.Run()
3. Collects catwalk_ prefixed functions and wraps them with meow_testing.Catwalk()
4. Appends meow_testing.Report() at the end of main()

Compiler Orchestration (compiler/)

The Compiler struct ties the pipeline together:

func (c *Compiler) CompileToGo(source, filename string) string {
    tokens := lexer.Lex(source, filename)
    parser := parser.New(tokens)
    prog, errs := parser.Parse()
    // ... error handling ...
    typeInfo := checker.Check(prog)
    gen := codegen.New()
    gen.SetTypeInfo(typeInfo)
    goCode, err := gen.Generate(prog)
    // ...
    return goCode
}

For Build and Run, the compiler:

1. Creates a temporary directory
2. Writes a go.mod and main.go with the generated code
3. Runs go build in the temp directory
4. Copies or executes the resulting binary

Runtime (runtime/meowrt/)

Value Interface

All Meow values implement:

type Value interface {
    Type() string     // "Int", "Float", "String", "Bool", etc.
    String() string   // String representation
    IsTruthy() bool   // Truthiness for conditions
}

Concrete Types

• Int — wraps int64
• Float — wraps float64
• String — wraps string
• Bool — wraps bool
• NilValue — singleton nil
• Func — wraps a Go function func(args ...Value) Value
• Furball — error value with Message string
• List — wraps []Value with helper methods
• Map — wraps map[string]Value
• Kitty — dynamic struct with TypeName, FieldNames, Fields map[string]Value

Operator Dispatch

Operators in operators.go use type switches to dispatch on operand types. All arithmetic requires same-type operands. Type mismatches panic with "Hiss! ...".

Error Convention

Runtime errors panic with strings matching "Hiss! <message>, nya~". Test assertion failures use a distinct testFailure panic type (not prefixed with “Hiss!”) so the test runner can distinguish assertion failures from runtime errors.