Re: emulate "sum type" pattern matching?

[Ricardo Wurmus <rekado@elephly.net>]

Chris Vine <vine35792468@gmail.com> writes:

> On Wed, 11 Mar 2020 19:58:04 +0000
> Sam Halliday <sam.halliday@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