Tilde Duck Typing

[Brother Bill]

It appears that ~ operator does duck typing.

a ~ b
If a and b are conforming in either direction, and have same shallow content, they are considered equivalent.
Please examine attachment and see if I am reading this correctly.
If so, ~ is a lot looser than = operator, which is subject to CATCALLs.

The = operator demands that a and b be of same type.

[rfo amalasoft.com]

The '=' sign requires that they refer to the same object.

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From: eiffel...@googlegroups.com <eiffel...@googlegroups.com> on behalf of Brother Bill <brother...@gmail.com>
Sent: Monday, June 10, 2024 6:57 PM
To: Eiffel Users <eiffel...@googlegroups.com>
Subject: [eiffel-users] Tilde Duck Typing

 

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[Brother Bill]

My bad.  I meant that "a.is_equal (b)" requires that a and b be of the same runtime type.

[Ulrich Windl]

You mean = being true means that both objects are the same (not just equal)? Your wording is confusing.

[javier...@gmail.com]

Please check the ECMA spec (https://ecma-international.org/wp-content/uploads/ECMA-367_2nd_edition_june_2006.pdf#page=135&zoom=100,76,297)

* 8.21.2 Syntax: Equality expressions 

Equality = Δ Expression Comparison Expression 

Comparison = Δ "=" | "/=" | "~" | "/~" 

* 8.21.3 Semantics: Equality Expression Semantics 

The Boolean_expression e ~ f has value true if and only if the values of e and f are both attached and such that e.is_equal (f) holds.

 The Boolean_expression e = f has value true if and only if the values of e and f are one of: 

            1 . Both void. 

            2 . Both attached to the same object with reference semantics.

            3. Both attached to objects with copy semantics, and such that e ~ f holds. 

Informative text 

The form with ~ always denotes object equality. 

The form with = denotes reference equality if applicable, otherwise object equality. Both rely, for object equality, on function is_equal — the version that can be redefined locally in any class to account for a programmer-defined notion of object equality adapted to the specific semantics of the class.  

-- Javier

[Brother Bill]

Checking out ECMA standard 2006.  I'll nibble at it day by day.  The definitive reference.  I might learn something...