Re: emulate "sum type" pattern matching?

[Chris Vine]

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