Chris Vine <vine35792...@gmail.com> writes:
> On Wed, 11 Mar 2020 19:58:04 +0000 > Sam Halliday <sam.halli...@gmail.com> wrote: >> Hi all, >> >> I have read the Guile manual as my introduction to Guile. I am very >> impressed at how mature this project is and was overwhelmed by the >> feature set, which seems to be on-par with heavily invested technologies >> like Java and the JVM. >> >> I am considering using Guile for a project because I love Emacs lisp and >> know it very well. Emacs lisp has some limitations that I feel Guile >> overcomes, e.g. multithreading, a superior regexp engine, a module >> system, and parsers. >> >> However, there is one feature that is critical to the development of the >> project and I was hoping to be able to implement it through a macro: sum >> type pattern matching. >> >> By that, I mean in the sense of Haskell sum types, which I understand >> are similar to C++ union types. Roughly translated into GOOP, and using >> Scala's encoding of sum types, this would look like record types that >> are all children of a superclass. I noticed that Guile has support for >> discovering all direct subclasses at runtime, but is this facility >> available at compiletime? >> >> An example of how I would want to use this feature can be described in >> terms of the XML calculator in the Guile Manual. >> https://www.gnu.org/software/guile/manual/html_node/sxml_002dmatch.html#Catamorphisms >> which looks like >> >> (define simple-eval >> (lambda (x) >> (sxml-match x >> [,i (guard (integer? i)) i] >> [(plus ,x ,y) (+ (simple-eval x) (simple-eval y))] >> [(times ,x ,y) (* (simple-eval x) (simple-eval y))] >> [(minus ,x ,y) (- (simple-eval x) (simple-eval y))] >> [(div ,x ,y) (/ (simple-eval x) (simple-eval y))] >> [,otherwise (error "simple-eval: invalid expression" x)]))) >> >> If the sxml-match was aware that it was matching over a superclass of >> plus, minus, times, div then the "otherwise" line would be redundant and >> (most importantly) if I were to forget to match over one of the >> subclasses I would get a compiler error. >> >> And that's basically my usecase in a nutshell: exhaustive pattern >> matching over a tree-like structure (an AST, in fact). But I'll have >> lots of different trees so I don't want to have to manually write a >> pattern match macro every time I define a "sum type"... although that >> said I do have some ideas on how to abstract that. But I don't really >> want to go down a lisp macro rabbit hole at the very beginning... > > guile's built-in pattern matcher (ice-9 match) enables you to match on > symbols, literals, pairs, lists, vectors and records, but I don't think > it enables you to match on GOOPS objects - someone may contradict me on > that, but at least I have never tried doing so (I don't like GOOPS nad > I rarely use it). You can match on anything as long as there is a predicate for it. For a GOOPS object that has a procedure “thing?” that returns #t when the argument is of the expected type you can match with (? thing? thing) You cannot, as far as I know, use match to know at compile time that the clauses are exhaustive. -- Ricardo
