in reply to Re^14: printing unitialized value of the 'do BLOCK' (summary)
in thread printing unitialized value of the 'do BLOCK'

The Perldocs are a bit fuzzy about the definitions of My normal understanding is that a program is a list of STATEMENTS.

They start after a semicolon and end with a semicolon, unless:

They mostly correspond to "lines" in speech, but line numbers are only recorded per statement.²

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's³ standard books showing that it's not an accident.

EXPRESSION is a general concept,

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 {}

²) multiple statements per line or multi-line statements break the mental analogy

³) ERRATA: rsFalse informed me that "Intermediate Perl was written by Schwartz and others, not including Larry."

Replies are listed 'Best First'.
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]
[reply]
[d/l]
[select]

        So Perl's if is an expression, especially when executed in a scalar or list context due to a surrounding do.

        No.

        Again, the condition of an if statement is defined to be an expression, so 

        if ( ... ) { ... }

        [download]

        isn't an expression because 

        if ( if ( ... ) { ... } ) { }

        [download]

        isn't valid.

        Furthermore, if any statement is an expression because any statement can be found at the end of the block of a do BLOCK, then the paragraph that's been quoted saying that the returned value is only defined for expressions wouldn't make sense.

        Your usage of the terms clearly differs from the documentation's.

[reply]
[d/l]
[select]

In Section Seekers of Perl Wisdom