Contexts and contextualizers

What are contexts and how to get into them

A context is needed, in many occasions, to interpret the value of a container. In Perl 6, we will use context to coerce the value of a container into some type or class, or decide what to do with it, as in the case of the sink context.

Sink

Sink is equivalent to void context, that is, a context in which we throw (down the sink, as it were) the result of an operation or the return value from a block. In general, this context will be invoked in warnings and errors when a statement does not know what to do with that value.

my $sub = -> $a { return $a² };
$sub# OUTPUT: «WARNINGS:␤Useless use of $sub in sink context (line 1)␤» 

You can force that sink context on Iterators, by using the sink-all method. Procs can also be sunk via the sink method, forcing them to raise an exception and not returning anything.

In general, blocks will warn if evaluated in sink context; however, gather/take blocks are explicitly evaluated in sink context, with values returned explicitly using take.

In sink context, an object will call its sink method if present:

sub foo {
    return [<a b c>does role {
        method sink { say "sink called" }
    }
}
foo
# OUTPUT: sink called 

Number

This context, and probably all of them except sink above, are conversion or interpretation contexts in the sense that they take an untyped or typed variable and duck-type it to whatever is needed to perform the operation. In some cases that will imply a conversion (from Str to Numeric, for instance); in other cases simply an interpretation (IntStr will be interpreted as Int or as Str).

Number context is called whenever we need to apply a numerical operation on a variable.

my $not-a-string="1                 ";
my $neither-a-string="3                        ";
say $not-a-string+$neither-a-string# OUTPUT: «4␤» 

In the code above, strings will be interpreted in numeric context as long as there is only some numbers and no other characters. It can have any number of leading or trailing whitespace, however.

Numeric context can be forced by using arithmetic operators such as + or -. In that context, the Numeric method will be called if available and the value returned used as the numeric value of the object.

my $t = True;
my $f = False;
say $t+$f;      # OUTPUT: «1␤» 
say $t.Numeric# OUTPUT: «1␤» 
say $f.Numeric# OUTPUT: «0␤» 
my $list= <a b c>;
say True+$list# OUTPUT: «4␤» 

In the case of listy things, the numeric value will be in general equivalent to .elems; in some cases, like Thread it will return an unique thread identifier.

String

In a string context, values can be manipulated as strings. This context is used, for instance, for coercing non-string values so that they can be printed to standard output.

    say $very-complicated-and-hairy-object# OUTPUT: something meaningful 

Or when smartmatching to a regular expression:

say 333444777 ~~ /(3+)/# OUTPUT: «「333」␤ 0 => 「333」␤» 

In general, the Str routine will be called on a variable to contextualize it; since this method is inherited from Mu, it is always present, but it is not always guaranteed to work. In some core classes it will issue a warning.

~ is the (unary) string contextualizer. As an operator, it concatenates strings, but as a prefix operator it becomes the string context operator.

my @array = [ [1,2,3], [4,5,6]];
say ~@array# OUTPUT: «1 2 3 4 5 6␤» 

This will happen also in a reduction context, when [~] is applied to a list

say [~] [ 35+6i, Set(<a b c>), [1,2,3] ]; # OUTPUT: «35+6ic a b1 2 3␤» 

In that sense, empty lists or other containers will stringify to an empty string:

say [~] [] ; # OUTPUT: «␤» 

Since ~ acts also as buffer concatenation operator, using it will have to check that every element is not empty, since a single empty buffer in string context will behave as a string, thus yielding an error.

say [~Buf.new(0x3,0x33), Buf.new(0x2,0x22);
# OUTPUT: «Buf:0x<03 33 02 22>␤» 

However,

my $non-empty = Buf.new(0x30x33);
my $empty = [];
my $non-empty-also = Buf.new(0x2,0x22);
say [~$non-empty$empty$non-empty-also;
# OUTPUT: «(exit code 1) Cannot use a Buf as a string, but you called the Stringy method on it 
# in block <unit> at /tmp/bO_heb6AS9 line 1␤␤» 

Since ~ is putting in string context the second element of this list, ~ is going to be using the second form that applies to strings, thus yielding the shown error. Simply making sure that everything you concatenate is a buffer will avoid this problem.

my $non-empty = Buf.new(0x30x33);
my $empty = Buf.new();
my $non-empty-also = Buf.new(0x2,0x22);
say [~$non-empty$empty$non-empty-also# OUTPUT: «Buf:0x<03 33 02 22>␤» 

In general, a context will coerce a variable to a particular type by calling the contextualizer; in the case of mixins, if the context class is mixed in, it will behave in that way.

my $described-number = 1but 'Unity in complex plane';
say $described-number# OUTPUT: «Unity in complex plane␤» 

but creates a mixin, which endows the complex number with a Str method. say puts into string context, that is, it calls Str, the string contextualizer, with the result shown above.