This repository contains a partial implementation of a macro expander in which macros can get stuck. The idea is that if a macro depends on information that will be the result of later macro expansion or type checking, it can pause its execution until that information has become available.
For a description of one way in which stuck macros might be useful, please see this talk.
Klister is written in Haskell, and is built with HPack and Cabal. You can build it like so:
hpack package.yaml
cabal v2-buildYou can use v2-install instead of v2-build to install the Klister executable and associated files to your home directory.
Run klister repl to be dropped into a read-eval-print loop where you can try out writing some Klister expressions.
klister run file.kl will evaluate the contents of file.kl.
The import form will search for modules in the same directory as the importing module, and in directories listed in the KLISTERPATH environment variable, a :-separated list of directories.
The macro expander itself is a set-of-scopes expander, based on Matthew Flatt's paper from POPL 2016 and described quite accessibly in his talk from Strange Loop.
Additionally, there is a module system patterned after Racket's.
This macro expander has a few differences:
- Rather than performing a depth-first traversal of the input syntax, expanding as it goes, our expander maintains a queue of expansion tasks. Tasks indicate the expression to be expanded as well as its resulting location in the final output. Dependency information is tracked in order to constrain the scheduling of expansion tasks.
- The core language does not coincide with the input language. Having an independent core language will hopefully allow us to overcome the overhead associated with recursive uses of
local-expand, as well as enabling a second, trusted type checking pass. - Type checking and macro expansion are interleaved. Every expansion step in an expression or pattern context knows what type the resulting program will have.
The type checker is a mostly-vanilla Hindley-Milner, based on Sestoft's description in Programming Language Concepts, extended with user-definable datatypes and Racket-style phase stratification of bindings. It uses Rémy's optimization of generalization, where type metavariables are assigned levels to avoid scanning the context at generalization time.
Initially, macros can get stuck by blocking until a signal has been sent. Signals are essentially just integers. Other macros can send signals, at which point the blocked macros get un-stuck. This is reminiscent of the blockOnMeta operation in Agda's reflection system. Eventually, we plan to replace this with the ability to block on the solution to a type unification variable.
"Klister" is Danish for "adhesive", and is also used to form words describing sticky things. And our macros get stuck.