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emulate "sum type" pattern matching?

From: Sam Halliday
Subject: emulate "sum type" pattern matching?
Date: Wed, 11 Mar 2020 19:58:04 +0000

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.
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...

Best regards,

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