[Python-ideas] Re: PEP 634-636: Mapping patterns and extra keys

[David Foster]

On 11/15/20 12:11 AM, Steven D'Aprano wrote:
It would be good if the PEP gave a survey of the practical experience of
other languages with pattern matching:
- are there languages which require an exact match, with no left over
   keys? what issues, if any, do users have with that choice?

- which languages ignore extra keys? do users of those languages
   consider this feature a bug, a wart, or a feature?

If useful I can volunteer to conduct such a survey. I expect to have 
some cycles this coming weekend.

I've completed my survey of how other languages use pattern matching to 
match Mapping-like and dict-like types, especially focusing on whether 
they ignore (𝔸) or disallow (𝔹) extra keys by default.

In general:

1. Dynamically-typed languages that support pattern matching have mostly 
experimental (†) pattern-matching implementations that all ignore extra 
keys by default (𝔸) rather than disallow them (𝔹). Some popular 
dynamically-typed languages don't support pattern-matching at all (✖️).
    - Erlang/Elixir are the only languages in this category with a
      non-experimental implementation.
    - Ruby and JavaScript both have experimental implementations only.

2. Statically-typed languages frequently provide pattern matching 
capabilities but they can't be used for Mapping-like or dict-like types 
(🤷‍♀️). Frequently the pattern matching provided *can* however be used 
on association lists (ordered lists of key-value tuples) or 
dataclass-like instances.

So it certainly seems at least the popular vote points toward ignoring 
extra keys by default rather than disallowing them by default, which is 
consistent with the current wording of PEP 634-636.

Below is the full survey summary:

    Key:
    * Pattern Match Approach
        𝔸 = Pattern-matches mappings/dicts, ignoring extra keys by default
        𝔹 = Pattern-matches mappings/dicts, disallowing extra keys by 
default
        🤷‍♀️ = Has pattern matching, but doesn't apply to a 
Mapping-like or dict-like types
        ✖️ = Has no pattern matching for algebraic data types (but 
might for strings)
    * Maturity
        † = Pattern-matching syntax is advertised as "experimental"

    Dynamically-typed languages:
    * Erlang
        𝔸
    * Elixir (derived from Erlang)
        𝔸
    * Ruby (2.7)
        𝔸†
    * JavaScript (TC39 draft)
        𝔸†
    * PHP
        ✖️
    * Lua
        ✖️
    * Python (current draft of PEP 634-636)
        𝔸†

    Statically-typed languages:
    * OCaml
        🤷‍♀️
    * Scala
        🤷‍♀️
    * Swift
        🤷‍♀️
    * Rust
        🤷‍♀️
    * Haskell
        🤷‍♀️
    * C++
        🤷‍♀️

--
David Foster | Seattle, WA, USA
Contributor to TypedDict support for mypy
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