Re^13: printing unitialized value of the 'do BLOCK' (summary)

So yes if and unless are effectively expressions in the form of statements

They are not.

$ perl -c -e'my $x = if (f()) { g(); } else { h(); }'
syntax error at -e line 1, near "= if"
-e had compilation errors.

$ perl -e'my $x = unless (f()) { g(); } else { h(); }'
syntax error at -e line 1, near "= unless"
-e had compilation errors.
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if statements and unless statements are statements, not expressions.

if (w/o else) and (the always short-circuiting) and are supposed to do the same

They're not, and they don't.

$ perl -c -e'my $x = ( f() and g() );'
-e syntax OK

$ perl -c -e'my $x = ( if (f()) { g() } );'
syntax error at -e line 1, near "( if"
-e had compilation errors.
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-MO=Concise shows opcodes, not operators. An if statement and an and expression often (but not always) use the and opcode, but that doesn't tell you anything relevant to this conversation.

Would you say an if statement in C and a while statement in C are the same thing because they both use the same branch opcode?

In short, you are begging the question. You are presuming an if statement does the same as an and expression to conclude that an if<c> statement does the same as an <c>and expression.

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Re^14: printing unitialized value of the 'do BLOCK' (summary) by rsFalse (Chaplain) on Dec 26, 2019 at 12:14 UTC
So it looks like they DO the same, except that one of them can't return a value unless enveloped by 'do{}', 'map{}...' etc.	[reply]
Re^15: printing unitialized value of the 'do BLOCK' (EXPRESSION vs TERM vs STATEMENT) by LanX (Saint) on Dec 26, 2019 at 14:48 UTC
The Perldocs are a bit fuzzy about the definitions of statement expression term My normal understanding is that a program is a list of STATEMENTS. They start after a semicolon and end with a semicolon, unless: neighboring curlies file borders ~~anonymous `[array]/{hash}` brackets~~ ° ... (Something I forgot, because only Perl can parse Perl) They mostly correspond to "lines" in speech, but line numbers are only recorded per statement.˛ A TERM is a sub-expression inside a statement. That's roughly everything you can put into round parens and terms can be nested. This corresponds to a branch or leaf in the op tree. In order to use a statement as a term, Perl offers the `do {BLOCK}` syntax. Other possibilities are `sub` and `eval` . Since statements are otherwise bound to be executed in `void` context (after a semicolon), we use them for their side effects and don't expect them to return (meaningful) values. But some do, like `if` . You've even cited a passage from one ~~of Larry's~~ł standard books showing that it's not an accident. EXPRESSION is a general concept, `perlsyn` explicitly calls statements "expressions" `perlop` lists "expressions in parenthesis" as terms and refers to `do` "being parsed as term". So I hope it's clearer now that the difference is mainly in how something is parsed into the syntax tree. I think part of the confusion comes from Larry using C syntax mixed with Lisp semantics. Disclaimer: I didn't have time to check all canonical Perldocs and sometimes it's not clear if an english word was used in its general meaning or to denote a Perl concept. ( like "Expressions are expressions", "and and or are operators", and so on...) Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery FootballPerl is like chess, only without the dice} footnote °) I probably misread perlop here, my tests don't allow to write a statement into `[]` or `{}` ˛) multiple statements per line or multi-line statements break the mental analogy ł) ERRATA: rsFalse informed me that "Intermediate Perl was written by Schwartz and others, not including Larry."	[reply] [d/l] [select]
Re^16: printing unitialized value of the 'do BLOCK' (EXPRESSION vs TERM vs STATEMENT) by ikegami (Patriarch) on Dec 27, 2019 at 06:16 UTC
The Perldocs are a bit fuzzy about the definitions of statement, expression, term A file is a sequence of statements. A block is a sequence of statements in curlies. A statement can be a flow control statement (`if`, `while`, etc), a simple statement (an expression used for its side-effects), as well as other things. An expression defines the syntax of code required to evaluate to one or more values. For example, an `if` statement's condition must be an expression. No one used the word "term". It's a parser rule that usually encompasses variables, literals and sub-expressions in parens. Not a very useful word. A TERM is a sub-expression inside a statement No. For example, `1+2` is not a term. An expression inside a statement is an expression (which contains terms). perlsyn explicitly calls statements "expressions" No, it doesn't. "The only kind of simple statement is an expression evaluated for its side-effects" is equivalent to "an expression evaluated for its side-effects is a simple statement." In other words, expressions are also statements. For example, `print("x")` is an expression (so you can do `if (print("x")) { ... }`), but it can also be used as a statement. So I hope it's clearer now that the difference is mainly in how something is parsed into the syntax tree. Both "statement" and "expression" are names of parser rules. As parser rules, there are two differences between them: The syntax they allow, and where they can be used.	[reply] [d/l] [select]
Re^17: printing unitialized value of the 'do BLOCK' (EXPRESSION vs TERM vs STATEMENT) by LanX (Saint) on Dec 27, 2019 at 17:20 UTC
Re^18: printing unitialized value of the 'do BLOCK' (EXPRESSION vs TERM vs STATEMENT) by ikegami (Patriarch) on Dec 28, 2019 at 13:57 UTC
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Re^15: printing unitialized value of the 'do BLOCK' (summary) by ikegami (Patriarch) on Dec 27, 2019 at 05:55 UTC
What an operator returns is a key aspect of what they do. But the point was that you can't argue that they must return the same thing because they otherwise have a similar effect.	[reply]

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