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). The other problem is that discriminated unions are a better fit with statically typed languages such as Haskell and the MLs; in dynamically typed languages there is a sense in which every variable name represents a union of unlimited extent but which enables its current type to be interrogated.