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List:       e-lang
Subject:    Re: [e-lang] e.enable.pattern-default
From:       "Mark S. Miller" <markm () caplet ! com>
Date:       2003-11-14 17:26:07
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At 08:32 AM 11/14/2003  Friday, Kevin Reid wrote:

> Does e.enable.pattern-default do what eprops.txt claims it does? I get a
> syntax error when I try to use the feature as described.
> 
> ? ["no" => "value"] =~ ["key" else ("absent") => matched]  
> # syntax error: 
> #   ["no" => "value"] =~ ["key" else ("absent") => matched]
> #                                  ^

My mistake. I changed the syntax without updating the documentation in 
eprops.txt. Try:

    ? pragma.enable("pattern-default")

    ? ["no" => "value"] =~ ["key" => matched := "absent"]
    # value: false

I'm not happy with either the old or new syntax, so suggestions would be 
great.


Reminder to all: Unless stated otherwise, language features that require a 
"pragma.enable(...)" are experimental and not an official part of the E 
language. There is no commitment that these features will continue to be 
available or behave the same way from one release to the next. The purpose 
of the pragma mechanism is to make available experimental features for us to 
play with, so we can discuss what features to adopt based on experience.

Neither the old nor new syntaxes for the above feature have ever been an 
official part of E. Their purpose is experimentation towards scope 
manipulation for a better module system (about which more later), and for 
keyword-based parameters:

    ? pragma.enable("pattern-default")
    ? pragma.enable("map-tail")

    ? def foo(i, "key" => matched := "absent") :String {
    >     ^matched + i
    > }
    # value: <foo>

    ? foo(3, "key" => "value", "foo" => "bar")
    # value: "value3"

    ? foo(3, "foo" => "bar")
    # value: "absent3"


Unlike (AFAIK) Python, keyword parameters would still not be supported by 
Kernel-E as a primitive concept. The above expands as follows:

    def foo(i, "key" => matched := "absent") :String {
        ^matched + i
    }

expands to

    def foo(i, ["key" => matched := "absent"]) :String {
        ^matched + i
    }

The ["key" => matched := "absent"] pattern above in turn expands to

    temp ? temp.extract("key", "absent") =~ [matched :any, _])

extract/2 is (too briefly) explained at
http://www.erights.org/javadoc/org/erights/e/elib/tables/EMap.html#extract(java.lang.Object,java.lang.Object)


The call

    foo(3, "key" => "value", "foo" => "bar")

expands to

    foo(3, ["key" => "value", "foo" => "bar"])


----------------------------------------
Text by me above is hereby placed in the public domain

        Cheers,
        --MarkM

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