InterpreterLib: A Library for Building Modular,

1. InterpreterLib: A Library for Building Modular, Extensible, and Composable InterpretersSystems Level Design Group (pweaver@ittc.ku.edu) Motivation Background • Functors • A functor is a type constructor f over which fmap is defined • fmap :: (a -> b) -> f a -> f b • Terms in an AST are functors, over which an evaluation • function can be mapped • Recursion Scheme • Interpreters typically follow a regular recursion scheme • All term arguments are evaluated first: • eval (Val n) = n • eval (Add term1 term2) = eval tm1 + eval tm2 • Can be generalized by folding a function, phi, over an AST: • phi (Val n) = n • phi (Add term1 term2) = term1 + term2 • eval = fold phi • Algebras • An Algebra defines a phi function for a particular language • Define a language as a combination of sub-languages. • Write an algebra for each sub-language. • Combine algebras to form an interpreter for the language • Modularity and Extensibilty • Interpreters should be made up of individual components • Components should be separate and should not depend • on each other • Components and features should be easy to add, without • requiring changes to other components in the interpreter • Executability • Interpreters should be executable • Interpreters written with InterpreterLib can be compiled • by ghc and executed • Provability • Programmer should be able to prove properties of • interpreters more easily, without manual induction • Abstraction • Lower-level details should be hidden from programmer • Haskell monads and InterpreterLib provide excellent • abstraction, doing a lot of work for the programmer! Details Example: SAFL to SECD • Fixed Points • A way to define recursive data structures non-recursively • newtype Fix f = In (f (Fix f)) • out (In x) = x • Catamorhpism • Folding is captured and generalized by a catamorphism • cata = phi ° fmap cata ° out • Inductive properties of a catamorphism are “free” • Algebra Compiler • User simply defines signature of language • ALGC generates haskell code for AST elements:: • Data structures for terms (functors) • Functions to build terms • Definition of fmap over the terms • Functions to generate algebras from phi functions • Future Work • Improvements to algebra compiler • Handle non-regular recursion schemes • Implement paramorphisms and anamorphism • Tool-chain written in InterpreterLib transforms a SAFL program to an SECD machine • Parser parses SAFL into an abstract syntax tree • Type checker and interpreter yield the types and values associated with AST elements • Type directed partial evaluator reifies the values to yield a normalized, partially evaluated AST • SAFL compiler transforms the AST into code for the SECD abstract machine and optionally synthesizes a netlist.