Mailman 3 Words rather than sigils in Structural Pattern Matching - Python-Dev

13 Nov 2020

      I have read a great deal of discussion on the pattern matching PEPs and
less formal discussions.  It is possible I have overlooked some post in all
of that, of course.

... OK, just saw Guido's "wait for new SC" comment, which I suppose applies
to this too :-).

One idea that I cannot recall seeing, but that seems to make sense to me
and fit with Python's feel is using a WORD to distinguish between a
variable value and a binding target.  That is, instead of a special symbol
prefixing or suffixing a name, either to indicate it is or is not a binding
target.  Of course, whether the extra word would be used for binding or for
NOT binding is a question still.

NOT_FOUND = 404
match http_code:
    case 200:
        print("OK document")
    case value NOT_FOUND:  # use the variable value
        print("Document not found")
    case OTHER_CODE:  # bind this name
        print("Other HTTP code", OTHER_CODE)

Of course, this would require a soft keyword, which is a disadvantage.
Going the other direction:

NOT_FOUND = 404
match http_code:
    case 200:
        print("OK document")
    case NOT_FOUND:  # use the variable value
        print("Document not found")
    case bind OTHER_CODE:  # bind this name
        print("Other HTTP code")

To me these read better than the punctuation characters.  But I guess some
folks have suggested enlisting 'as', which is a word, of course.

-- 
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Words rather than sigils in Structural Pattern Matching

David Mertz

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