On Mon, Nov 23, 2020 at 08:44:21PM +0000, David Mertz wrote:
> Basically, I agree matching/destructuring is a powerful idea. But I also
> wonder how much genuinely better it is than a library that does not require
> a language change. For example, I could create a library to allow this:
>
> m = Matcher(arbitrary_expression)
> if m.case("StringNode(s)"):
> process_string(m.val)
> elif m.case("[a, 5, 6, b]"):
> process_two_free_vars(*m.values)
> elif m.case("PairNone(a, b)"):
> a, b = m.values
> process_pair(a, b)
> elif m.case("DictNode"):
> foo = {key, process_node(child_node) for key, child_node in
> m.values.items()}
Look at all those strings. So much for syntax highlighting and
compile-time syntax checking. Who needs them anyway?
# Perfectly legal syntax.
if m.case("StringNode[s)"):
...
elif m.case("[a, 5 6, b"):
...
It's a good thing we already have comprehensions, because if we didn't
have them, people would argue that they aren't necessary, we can just
write a `comprehension` function that takes the loop expression as a
string:
# [(a + int(''.join(b)))*c) for a, b, c in items]
comprehension("(a + int(''.join(b)))*c)", items]
Imagine how silly we would be to dedicate actual syntax to
comprehensions, when we can write a library to do it. It's all this
syntactic sugar (comprehensions, decorators, f-strings, async, with
statements, etc) that is killing Python.
*wink*
> I don't disagree that the pattern mini-language looks nice as syntax. But
> there's nothing about that mini-language that couldn't be put in a library
> (with the caveat that patterns would need to be quoted in some way).
One thing that we get with pattern matching syntax is the absense of
certain *misfeatures*.
# Oops, I forgot to use `elif`, now I have fall-through semantics
# which is widely agreed to be a bad idea.
m = Matcher([4, 5, 6, 7])
if m.case("[a, 5, 6, b]"):
print("first case")
if m.case("[4, a, b, 7]"):
print("second case")
As the library author, I hope you are prepared for many, many bug
reports about why people's code behaves differently if they use `if`
compared to `elif`.
Another misfeature: the ability to scatter your pattern matching cases
all over the place.
m = Matcher(expression)
do_this()
do_that()
if m.case(something):
process("this case")
do_another_thing()
class Spam:
# snip five pages of code
if m.case(something_else):
print("Did you forget we were inside a match pseudo-block?")
Objection: "But coders won't do that!"
No, *sensible* coders won't do that. With pattern matching syntax, even
the other sort of coder can't do that.
Objection: "But they could put five pages of code inside a case block
too."
True, but only if it is *conditional* to that specific case. You can't
mix unconditional code and cases. And the extra indentation will hint
that something odd is going on.
Another misfeature: the ability to modify the value being matched in the
middle of the pattern matching pseudo-block.
m = Matcher(something)
if m.case(spam):
process("case 1")
m = Matcher(another_thing)
if m.case(eggs):
process("case 2")
People can write obfuscated, confusing, poor-quality code with anything,
but syntax can limit their opportunities to do so.
--
Steve
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