bhc-intern

String interning for efficient symbol handling in the Basel Haskell Compiler.

Overview

This crate provides interned strings (symbols) that enable O(1) equality comparisons and reduced memory usage for repeated strings. A global interner ensures that each unique string is stored only once.

Key Types

Usage

use bhc_intern::{Symbol, Ident, kw};

// Intern a string
let s1 = Symbol::intern("hello");
let s2 = Symbol::intern("hello");

// O(1) comparison (just compares indices)
assert_eq!(s1, s2);

// Get the string value
assert_eq!(s1.as_str(), "hello");

// Use pre-interned keywords
kw::intern_all(); // Optional: pre-intern for better performance
assert_eq!(*kw::LET, "let");
assert_eq!(*kw::WHERE, "where");

// Create identifiers
let id = Ident::from_str("myFunction");
println!("{}", id); // prints: myFunction

Pre-interned Keywords

The kw module provides pre-interned symbols for common Haskell keywords:

Haskell Keywords: case, class, data, deriving, do, else, forall, foreign, if, import, in, infix, infixl, infixr, instance, let, module, newtype, of, qualified, then, type, where

BHC Extensions: lazy, strict, profile, edition

Common Types: Int, Float, Double, Bool, Char, String, ()

Common Constructors: True, False, Just, Nothing, Left, Right

Performance

• Interning: O(1) average case (hash lookup), O(n) for new strings
• Comparison: O(1) - just compares integer indices
• Memory: Each unique string stored once, symbols are just 4 bytes

Thread Safety

The global interner is thread-safe using RwLock. The fast path (already interned) only requires a read lock.

Design Notes

• Symbols are Copy and very cheap to pass around
• The interner leaks memory intentionally (strings live forever)
• Use kw::intern_all() at startup for predictable performance

Related Crates

• bhc-ast - Uses symbols for identifiers in the AST
• bhc-hir - Uses symbols for names in HIR
• bhc-core - Uses symbols for variable names in Core IR