Hi all,
I'm playing around with core.match and trying to extend it to some
custom types in terms of extending the IMatchLookup protocol on those.
But that's not quite what I need. IMatchLookup only specifies how to
access some value out of my custom types, and in the `match' clauses
those values are compared by equality to the values specified there.
In my patterns, I want to have a :+type key with a value that's
basically an interface name given as a symbol. The pattern should
match, if the class of the object implements that interface (or an
extended interface thereof) directly or indirectly. So that would be my
ideal user interface:
--8<---------------cut here---------------start------------->8---
(match [obj]
[{:+type 'Cat :mice 4}] :1
[{:+type 'Dog :cats 3}] :2
[{:+type 'Mammal}] :3
:else :nope)
--8<---------------cut here---------------end--------------->8---
For example, a Cat object with 7 mice would miss the first row but match
the third one, because Cat is a specialization of Mammal.
I can still achieve what I want by introducing an :+obj key returning
the actual object in favour of :+type and then use guards to do the
dispatch on type, but that puts the burden on the user. `type-matcher'
is a custom function that gets some type specification and returns a
predicate that tests if a given object conforms to the type
specification.
--8<---------------cut here---------------start------------->8---
(match [node]
[{:+obj (a :when (core/type-matcher node 'Cat)) :mice 4}] :1
[{:+obj (b :when (core/type-matcher node 'Dog)) :cats 3}] :2
[{:+obj (c :when (core/type-matcher node 'Mammal))}] :3
:else :nope)
--8<---------------cut here---------------end--------------->8---
Clearly, that's much worse than the idealized user interface above. So
is there a way to do what I want? Maybe even core.match could check, if
IMatchLookup/val-at* returned a function, and if so, simply apply it to
the value specified in the pattern as match test instead of the
hard-coded `='?
I have some more use-cases where I would be very benefitical to return
predicates for matching from val-at*. For example, if some property of
my object is a collection, I'd very much like to have a key whose value
should be contained in the collection.
Thanks for any hints,
Tassilo
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