Eval

This is an implementation for a small, strongly-typed functional language with parametric polymorphism, user-defined datatypes, pattern matching, and general recursion. The implementation uses no dependencies beyond the OCaml standard library.

Done:

• Definition of the syntax. See syntax.ml.
• Evaluation of terms, including pattern-matching and recursion, and top-level declarations. See eval.ml.
• Pretty-printing of terms. See pretty.ml.
• Unification. See unify.ml.
• Typechecking. See typecheck.ml.
• Some example programs. See examples/ski.ev.
• External syntax. This is the syntax recognized by the parser.
• Scopechecking to translate external syntax to internal syntax. This pass ensures that all references are valid (constructors, other functions, types, variables, etc.)
• Parsing. The parser is a handwritten recursive descent parser using combinators. The error messages aren't great!
• Bytecode compilation. The bytecode is described in bytecode.ml, for a two-stack virtual machine (one for locals/parameters and one for the call stack)

Todo:

• tail-call optimization
• optimized pattern-matching compilation (current strategy is linear / naive)
• efficient interpreter implemented in C++ or Rust
• recursive local bindings (only top-level recursive functions work at the moment)

Structure

• Syntax: definitions of different ASTs
  • synext.ml external syntax: this is close to what the user writes
  • synint.ml internal syntax: what we typecheck
  • synclo.ml closed syntax: an intermediate representation generated by closure conversion.
• Loc: source locations
• Pretty: pretty-printing of internal syntax, closed syntax, and bytecode instructions.
• Parser: hand-written parser combinators that generate external syntax from a string.
• Scopecheck: converts external syntax to internal syntax, disambiguates names (locals vs references), scope-checks all identifiers.
• TMVar: unification variables, aka "type metavariables" (TMVars); type substitutions
• Unify: unification of types
• Typecheck: Hindley-Milner type inference using unification
• Eval: evaluator for internal syntax, turns internal syntax into a value.
• Close: closure conversion hoists function bodies occurring within functions to the top-level and replaces them with an MkClo node.
• Bytecode: description of the bytecode instructions emitted by compilation.
• Compile: compiles closed syntax into bytecode instructions.
• Interpret: bytecode interpreter.

The main flow of data is: text -> Parser -> Scopecheck -> Typecheck -> Close

Bugs

Currently, applications of constructors, statically known functions, and primitive operations are automatically eta-expanded, but that eta-expansion introduces new abstractions without appropriately adjusting the indices of bound variables, so it produces bogus code. Manually eta-expand everything as a workaround for now.