Match with symbols

[jakesnell]

I can't seem to figure out how get match to work with symbols. Here is my attempt with Hy 0.25.0 using CPython(main) 3.11.0:

=> (match 'paper
...   'rock 'win
...   'scissors 'lose
...   'paper 'draw)
Traceback (most recent call last):
  File "stdin-d863505ec44612e7c4bb8553e11080f06b1530c4", line 1, in <module>
    (match 'paper

NameError: name 'quote' is not defined

It's not that quote is completely missing, because I can use it to define symbols outside of a match:

=> 'rock
'rock
=> (quote rock)
'rock

On the other hand, the same match works for strings:

=> (match "paper"
...   "rock" "win"
...   "scissors" "lose"
...   "paper" "draw")
"draw"

Any ideas on what I'm doing wrong?

[Kodiologist]

Python limits what is allowed as a pattern in a match statement, and Hy doesn't try to extend this. In particular, Hy won't try to expand macro calls in a pattern, and 'rock is just syntactic sugar for calling the macro quote. I don't know what your real situation is, but I bet you'd be happier with Hyrule's case or branch.

[jakesnell]

Hyrule's case looks like what I need! Thanks for the tip and for the explanation regarding match.

[wrobell]

@Kodiologist I believe the following is related, so asking here.

(match (hy.models.Symbol "a")
  (hy.models.Symbol "a") (print "hey a")
  _ (print "not a"))

Above prints not a and is bit confusing, because

(match 'a
  (hy.models.Symbol a) (print "hey a" a)
  _ (print "not a")) =

prints hey a a.

We can match against any symbol object, but we cannot match against a specific symbol value. Would that be a bug?

[Kodiologist]

Good question. I think this is a bug, related to how we define Symbol.__new__, but I'll look into it.

[Kodiologist]

No, honestly, it's not clear what should happen. The semantics of match are messy in their design and not documented very well. How should Symbol be treated given that it's a subclass of str, rather than having a str as an attribute? I guess the moral of the story is that match stinks and you shouldn't use it.

[wrobell]

This seems to work as expected:

from hy.models import Symbol as HySymbol

class Symbol(str):  # <- derive from HySymbol and stops working
    pass

match Symbol('a'):
    case Symbol('b') as a:
        print('hey b', a)
    case Symbol('a') as a:  # match here
        print('hey a', a)
    case _:
        print('not ok')

I will try to take a look at hy.models.Symbol later.

[wrobell]

Please see #2597