Re^16: printing unitialized value of the 'do BLOCK' (EXPRESSION vs TERM vs STATEMENT)

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.

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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
I think we mostly agree. I read up the WP pages on statement and expression and it becomes clear that it's far easier to make a distinction in languages like C. But Perl took also a lot from Lisp, where everything is an expression returning a value. Quote: "In languages that mix imperative and functional styles, such as the Lisp family, the distinction between expressions and statements is not made: even expressions executed in sequential contexts solely for their side effects and whose return values are not used are considered 'expressions'" So Perl's `if` is an expression, especially when executed in a scalar or list context due to a surrounding `do`. And it's also a statement, because it must follow a semicolon or a blocks start ("sequential context") it is mainly used for flow control ("side effects") it's return value is rarely used. Update OTOH does `A and B` (= `do { if (A) {B} }` ) have the side-effect of flow-control, because it's short-circuiting. Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}	[reply] [d/l] [select]
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
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]
Re^19: printing unitialized value of the 'do BLOCK' (EXPRESSION vs TERM vs STATEMENT) by LanX (Saint) on Dec 28, 2019 at 17:20 UTC
> Your usage of the terms clearly differs from the documentation's. Depending on the definition you take I'd say according to `perlglossary` it must be parsable without surrounding `do {}` expression Anything you can legally say in a spot where a value is required. Typically composed of literals, variables, operators, functions, and subroutine calls, not necessarily in that order. But according to WP expression, the defining characteristic is returning a value An expression in a programming language is a combination of one or more constants, variables, operators, and functions that the programming language interprets (according to its particular rules of precedence and of association) and computes to produce ("to return", in a stateful environment) another value. I'd compromise on "statement which can be used like an expression to produce values". Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}	[reply] [d/l]
Re^20: printing unitialized value of the 'do BLOCK' (EXPRESSION vs TERM vs STATEMENT) by ikegami (Patriarch) on Dec 28, 2019 at 17:33 UTC
Re^21: printing unitialized value of the 'do BLOCK' (EXPRESSION vs TERM vs STATEMENT) by LanX (Saint) on Dec 28, 2019 at 17:53 UTC
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Re^16: printing unitialized value of the 'do BLOCK' (EXPRESSION vs TERM vs STATEMENT)

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