Scalar and List Context

This is the most important section in this lesson.  In fact, it's the most important section in the entire tutorial.  In fact, it wouldn't be an exaggeration to say that your entire career in using Perl will depend upon understanding this section.  So if you've gotten away with skimming the text up to this point, this is where you should really pay attention.

That's not to say that this section is in any way difficult to understand.  It's actually a simple idea:  a given expression may mean different things depending upon where it appears.  This is nothing new to you; it happens all the time in natural languages.  For example, in English,¹ suppose someone asked you what the word "read"² means.  It has different meanings depending upon how it's used.  You can't identify the meaning, until you know the context.

The context refers to where an expression is found.  As Perl is parsing your expression, it always expects either a scalar value or a list value.³   What Perl expects is called the context of the expression.⁴

	5 + something	# The something must be a scalar
	sort something	# The something must be a list

Even if something is the exact same sequence of characters, in one case it may give a single, scalar value, while in the other, it may give a list.⁵

Expressions in Perl always return the appropriate value for their context.  For example, how about the "name"⁶ of an array.  In a list context, it gives the list of elements.  But in a scalar context, it returns the number of elements in the array:

	@people = qw( fred barney betty );
	@sorted = sort @people;	# list context:  barney, betty, fred
	$number = 5 + @people;	# scalar context:  5 + 3 gives 8

Even ordinary assignment (to a scalar or a list) causes different contexts:

	@list = @people;	# a list of three people
	$n = @people;		# the number 3

But please don't jump to the conclusion that scalar context always gives the number of elements that would have been returned in list context.  Most list-producing expressions ⁷ return something much more interesting than that.

[1] If you aren't a native speaker of English, this analogy may not be obvious to you.  But context sensitivity happens in every spoken language, so you may be able to think of an example in your own language.

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[2] Or maybe they were asking what the word "red" means, if they were speaking rather than writing a book.  It's ambiguous either way.  As Douglas Hofstadter said, no language can express every thought unambiguously, especially this one.

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[3] Unless, of course, Perl is expecting something else entirely.  There are other contexts that aren't covered here.  In fact, nobody knows how many contexts Perl uses; the biggest brains in all of Perl haven't agreed on an answer to that yet.

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[4] This is no different than what you are used to in human languages.  If I make a grammatical mistake, you notice it right away, because you expect certain words in places certain.  Eventually, you'll read Perl this way, too, but at first you have to think about it.

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[5] The list may be just one element long, of course.  It could also be empty, or it could have any number of elements.

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[6] Well, the true name of the array @people is just people.  The @-sign is just a qualifier.

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[7] But with regard to the point of this section, there's no difference between a "list- producing" expression and a "scalar-producing" one; any expression can produce a list or a scalar, depending upon context.  So when we say "list-producing expressions" we mean expressions that are typically used in a list context and that therefore might suprise you when they're used unexpectedly in a scalar context (like reverse or @fred).

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