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Depending on the context we get either a scalar or an array. Ok so far.
But in the following: 

print reverse <STDIN>;

Why do I get a list context? I mean reverse according to doc is either a list or a scalar context. So is print.

Prints a string or a list of strings. Returns true if successful.

So is STDIN. So why does STDIN here retrieve lines until EOF and not just collect the first line?

TLP
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Cratylus
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    The possible arguments for print are either FILEHANDLE LIST, FILEHANDLE, LIST or nothing. So except for the file handle, print will always impose list context on it's arguments. If you give it a scalar or a string literal, `print` still imposes list context on those, even though it doesn't affect the program. – tauli Apr 11 '13 at 13:47
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    Keep reading the perldoc for `print`: `... Because print takes a LIST, anything in the LIST is evaluated in list context, including any subroutines whose return lists you pass to "print". ...` – mob Apr 11 '13 at 13:59
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    The docs for `print` aren't relevant. `STDIN` is being passed to `reverse`, not to `print`. (`print` gets the output of `reverse`.) – Dave Sherohman Apr 11 '13 at 14:01
  • @DaveSherohman:But where does the doc for reverse talk about its arguments? – Cratylus Apr 11 '13 at 14:06
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    The first two words in `perldoc reverse` are "reverse LIST". – Dave Sherohman Apr 11 '13 at 14:07
  • Also see http://stackoverflow.com/questions/5170720/perl-scalar-vs-list-context?rq=1 for a good discussion on scalar vs list context. – devnull Apr 11 '13 at 14:13

3 Answers3

7

You seem to be conflating two independent things:

  • An operator is evaluated in list, scalar or void context.
  • An operator decides in which context its operands are evaluated.

The operands of reverse are always evaluated in list context.

reverse LIST

So <STDIN> will be evaluated in list context.

Like all operators that can return something other than a scalar, reverse behaves differently in scalar context and in list context.

The operands of print are always evaluated in list context.

print LIST

So reverse will be evaluated in list context. That means it will reverse the order of its operands. It won't reverse the order of the characters of each operand, and it won't concatenate the list.

ikegami
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3

Actually print always prints a list of strings. You can pass it a scalar, but it will behave as if it's a list of one.

So in this case reverse knows it's called in list context and thus reverses its list argument.

Rob I
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  • I saw that link.It says `Prints a string or a list of strings.` So could be scalar or list I think. – Cratylus Apr 11 '13 at 13:38
  • Note the arguments in the perldoc - the "LIST" argument rather than an "EXPR" argument (reference `len`). So it always treats arguments as a list, regardless of how many. – Rob I Apr 11 '13 at 13:42
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    Actually, even though the semantics of `reverse` change depending on the context it is in `reverse` itself will always impose list context on it's arguments. – tauli Apr 11 '13 at 13:43
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    @tauli:But where does the doc for reverse talk about its arguments? – Cratylus Apr 11 '13 at 14:07
  • @Cratylus right in the first line of `perldoc -f reverse` it says `reverse LIST` which means that reverse takes a list as an argument. – tauli Apr 11 '13 at 14:49
2

print reverse <STDIN>; is essentially the same as:

@lines = <STDIN>;
@reversed_lines = reverse @lines;
print @reversed_lines;

Saying $line = <STDIN>; would read one line from STDIN whereas @line = <STDIN>; reads a list from STDIN.

The argument list of print is always a list context. Line context can be enforced by using concatenation operator: print reverse <STDIN> . ""; would read a line and not a list.

devnull
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  • But why is it interpreted as `@lines=` in the compact form of my OP? This is my question – Cratylus Apr 11 '13 at 13:53
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    @Cratylus: Because the argument to `reverse` is always a list. – Hasturkun Apr 11 '13 at 13:57
  • @Cratylus: The argument list of `print` is always a list context. You can enforce the line context by using concatenation operator: `print reverse . "";` would read a *line*. – devnull Apr 11 '13 at 14:05
  • @devnull Or you could use the more appropriate `scalar` function: `print reverse scalar `. Or even better, use an assignment: `while (my $line = ) { print reverse $line }` – TLP Apr 11 '13 at 14:20
  • Yes, I didn't mention `scalar`. The other one is mentioned in the answer above. – devnull Apr 11 '13 at 14:28