When learning a programming language, possibly with the background of being familiar with another programming language, there are always some things that can surprise you and might cost valuable time in debugging and discovery.

This document aims to show common misconceptions in order to avoid them.

During the making of Raku great pains were taken to get rid of warts in the syntax. When you whack one wart, though, sometimes another pops up. So a lot of time was spent finding the minimum number of warts or trying to put them where they would rarely be seen. Because of this, Raku's warts are in different places than you may expect them to be when coming from another language.

Variables and constants

Constants are computed at compile time

Constants are computed at compile time, so if you use them in modules keep in mind that their values will be frozen due to precompilation of the module itself:

# WRONG (most likely): 
unit module Something::Or::Other;
constant $config-file = "config.txt".IO.slurp;

The $config-file will be slurped during precompilation and changes to config.txt file won't be re-loaded when you start the script again; only when the module is re-compiled.

Avoid using a container and prefer binding a value to a variable that offers a behavior similar to a constant, but allowing the value to get updated:

# Good; file gets updated from 'config.txt' file on each script run: 
unit module Something::Or::Other;
my $config-file := "config.txt".IO.slurp;

Assignment of Nil can produce a different value, usually Any

Actually, assignment of Nil to a variable reverts the variable to its default value. For example,

my @a = 4, 8, 15, 16;
@a[2] = Nil;
say @a; # OUTPUT: «[4 8 (Any) 16]␤» 

In this case, Any is the default value of an Array element.

You can purposefully assign Nil as a default value:

my %h is default(Nil) = a => Nil;
say %h; # OUTPUT: «Hash %h = {:a(Nil)}␤» 

Or bind a value to Nil if that is the result you want:

@a[3] := Nil;
say @a; # OUTPUT: «[4 8 (Any) Nil]␤» 

This trap might be hidden in the result of functions, such as matches:

my $result2 = 'abcdef' ~~ / dex /;
say "Result2 is { $result2.^name }"; # OUTPUT: «Result2 is Any␤» 

A Match will be Nil if it finds nothing; however assigning Nil to $result2 above will result in its default value, which is Any as shown.

Using a block to interpolate anon state vars

The programmer intended for the code to count the number of times the routine is called, but the counter is not increasing:

sub count-it { say "Count is {$++}" }
count-it;
count-it;
 
# OUTPUT: 
# Count is 0 
# Count is 0 

When it comes to state variables, the block in which the vars are declared gets cloned —and vars get initialized anew— whenever that block's block is re-entered. This lets constructs like the one below behave appropriately; the state variable inside the loop gets initialized anew each time the sub is called:

sub count-it {
    for ^3 {
        state $count = 0;
        say "Count is $count";
        $count++;
    }
}
 
count-it;
say "…and again…";
count-it;
 
 
# OUTPUT: 
# Count is 0 
# Count is 1 
# Count is 2 
# …and again… 
# Count is 0 
# Count is 1 
# Count is 2 

The same layout exists in our buggy program. The { } inside a double-quoted string isn't merely an interpolation to execute a piece of code. It's actually its own block, which is just as in the example above gets cloned each time the sub is entered, re-initializing our state variable. To get the right count, we need to get rid of that inner block, using a scalar contextualizer to interpolate our piece of code instead:

sub count-it { say "Count is $($++)" }
count-it;
count-it;
 
# OUTPUT: 
# Count is 0 
# Count is 1 

Alternatively, you can also use the concatenation operator instead:

sub count-it { say "Count is " ~ $++ }

Using set subroutines on Associative when the value is falsy

Using (cont), ∋, ∌, (elem), (elem), or ∉ on classes implementing Associative will return False if the value of the key is falsy:

enum Foo «a b»;
say Foo.enums ∋ 'a';
 
# OUTPUT: 
# False 

Instead, use :exists:

enum Foo «a b»;
say Foo.enums<a>:exists;
 
# OUTPUT: 
# True 

Blocks

Beware of empty "blocks"

Curly braces are used to declare blocks. However, empty curly braces will declare a hash.

$ = {say 42;} # Block 
$ = {;}       # Block 
$ = {…}       # Block 
$ = { }       # Hash 

You can use the second form if you effectively want to declare an empty block:

my &does-nothing = {;};
say does-nothing(33); # OUTPUT: «Nil␤»

Objects

Assigning to attributes

Newcomers often think that, because attributes with accessors are declared as has $.x, they can assign to $.x inside the class. That's not the case.

For example

class Point {
    has $.x;
    has $.y;
    method double {
        $.x *= 2;   # WRONG 
        $.y *= 2;   # WRONG 
        self;
    }
}
 
say Point.new(x => 1, y => -2).double.x
# OUTPUT: «Cannot assign to an immutable value␤» 

the first line inside the method double is marked with # WRONG because $.x, short for $( self.x ), is a call to a read-only accessor.

The syntax has $.x is short for something like has $!x; method x() { $!x }, so the actual attribute is called $!x, and a read-only accessor method is automatically generated.

Thus the correct way to write the method double is

method double {
    $!x *= 2;
    $!y *= 2;
    self;
}

which operates on the attributes directly.

BUILD prevents automatic attribute initialization from constructor arguments

When you define your own BUILD submethod, you must take care of initializing all attributes by yourself. For example

class A {
    has $.x;
    has $.y;
    submethod BUILD {
        $!y = 18;
    }
}
 
say A.new(x => 42).x;       # OUTPUT: «Any␤» 

leaves $!x uninitialized, because the custom BUILD doesn't initialize it.

Note: Consider using TWEAK instead. Rakudo supports TWEAK method since release 2016.11.

One possible remedy is to explicitly initialize the attribute in BUILD:

submethod BUILD(:$x) {
    $!y = 18;
    $!x := $x;
}

which can be shortened to:

submethod BUILD(:$!x) {
    $!y = 18;
}

Whitespace

Whitespace in regexes does not match literally

say 'a b' ~~ /a b/; # OUTPUT: «False␤» 

Whitespace in regexes is, by default, considered an optional filler without semantics, just like in the rest of the Raku language.

Ways to match whitespace:

• \s to match any one whitespace, \s+ to match at least one

• ' ' (a blank in quotes) to match a single blank

• \t, \n for specific whitespace (tab, newline)

• \h, \v for horizontal, vertical whitespace

• .ws, a built-in rule for whitespace that oftentimes does what you actually want it to do

• with m:s/a b/ or m:sigspace/a b/, the blank in the regexes matches arbitrary whitespace

Ambiguities in parsing

While some languages will let you get away with removing as much whitespace between tokens as possible, Raku is less forgiving. The overarching mantra is we discourage code golf, so don't scrimp on whitespace (the more serious underlying reason behind these restrictions is single-pass parsing and ability to parse Raku programs with virtually no backtracking).

The common areas you should watch out for are:

Block vs. Hash slice ambiguity

# WRONG; trying to hash-slice a Bool: 
while ($++ > 5){ .say }

# RIGHT: 
while ($++ > 5) { .say }
 
# EVEN BETTER; Raku does not require parentheses there: 
while $++ > 5 { .say }

Reduction vs. Array constructor ambiguity

# WRONG; ambiguity with `[<]` metaop: 
my @a = [[<foo>],];

# RIGHT; reductions cannot have spaces in them, so put one in: 
my @a = [[ <foo>],];
 
# No ambiguity here, natural spaces between items suffice to resolve it: 
my @a = [[<foo bar ber>],];

Less than vs. Word quoting/Associative indexing

# WRONG; trying to index 3 associatively: 
say 3<5>4

# RIGHT; prefer some extra whitespace around infix operators: 
say 3 < 5 > 4

Exclusive sequences vs. sequences with Ranges

See the section on operator traps for more information about how the ...^ operator can be mistaken for the ... operator with a ^ operator immediately following it. You must use whitespace correctly to indicate which interpretation will be followed.

Captures

Containers versus values in a capture

Beginners might expect a variable in a Capture to supply its current value when that Capture is later used. For example:

my $a = 2; say join ",", ($a, ++$a);  # OUTPUT: «3,3␤» 

Here the Capture contained the container pointed to by $a and the value of the result of the expression ++$a. Since the Capture must be reified before &say can use it, the ++$a may happen before &say looks inside the container in $a (and before the List is created with the two terms) and so it may already be incremented.

Instead, use an expression that produces a value when you want a value.

my $a = 2; say join ",", (+$a, ++$a); # OUTPUT: «2,3␤» 

Or even simpler

my $a = 2; say  "$a, {++$a}"; # OUTPUT: «2, 3␤» 

The same happens in this case:

my @arr;
my ($a, $b) = (1,1);
for ^5 {
    ($a,$b) = ($b, $a+$b);
    @arr.push: ($a, $b);
    say @arr
};

Outputs «[(1 2)]␤[(2 3) (2 3)]␤[(3 5) (3 5) (3 5)]␤.... $a and $b are not reified until say is called, the value that they have in that precise moment is the one printed. To avoid that, decontainerize values or take them out of the variable in some way before using them.

my @arr;
my ($a, $b) = (1,1);
for ^5 {
    ($a,$b) = ($b, $a+$b);
    @arr.push: ($a.item, $b.item);
    say @arr
};

With item, the container will be evaluated in item context, its value extracted, and the desired outcome achieved.

Cool tricks

Raku includes a Cool class, which provides some of the DWIM behaviors we got used to by coercing arguments when necessary. However, DWIM is never perfect. Especially with Lists, which are Cool, there are many methods that will not do what you probably think they do, including contains, starts-with or index. Please see some examples in the section below.

Strings are not Lists, so beware indexing

In Raku, strings are not lists of characters. One cannot iterate over them or index into them as you can with List, despite the name of the .index method.

Lists become strings, so beware .index()ing

List inherits from Cool, which provides access to .index method. Because of the way .index coerces a List into a strings, this can sometimes appear to be returning the index of an element in the list, but that is not how the behavior is defined.

my @a = <a b c d>;
say @a.index(‘a’);    # 0 
say @a.index('c');    # 4 -- not 2! 
say @a.index('b c');  # 2 -- not undefined! 
say @a.index(<a b>);  # 0 -- not undefined! 

These same caveats apply to .rindex.

Lists become strings, so beware .contains()

Similarly, .contains does not look for elements in the list.

my @menu = <hamburger fries milkshake>;
say @menu.contains('hamburger');            # True 
say @menu.contains('hot dog');              # False 
say @menu.contains('milk');                 # True! 
say @menu.contains('er fr');                # True! 
say @menu.contains(<es mi>);                # True! 

If you actually want to check for the presence of an element, use the (cont) operator for single elements, and the superset and strict superset operators for multiple elements.

my @menu = <hamburger fries milkshake>;
say @menu (cont) 'fries';                   # True 
say @menu (cont) 'milk';                    # False 
say @menu (>) <hamburger fries>;            # True 
say @menu (>) <milkshake fries>;            # True (! NB: order doesn't matter) 

If you are doing a lot of element testing, you may be better off using a Set.

Numeric literals are parsed before coercion

Experienced programmers will probably not be surprised by this, but Numeric literals will be parsed into their numeric value before being coerced into a string, which may create nonintuitive results.

say 0xff.contains(55);      # True 
say 0xff.contains(0xf);     # False 
say 12_345.contains("23");  # True 
say 12_345.contains("2_");  # False 

Getting a random item from a List

A common task is to retrieve one or more random elements from a collection, but List.rand isn't the way to do that. Cool provides rand, but that first coerces the List into the number of items in the list, and returns a random real number between 0 and that value. To get random elements, see pick and roll.

my @colors = <red orange yellow green blue indigo violet>;
say @colors.rand;       # 2.21921955680514 
say @colors.pick;       # orange 
say @colors.roll;       # blue 
say @colors.pick(2);    # yellow violet  (cannot repeat) 
say @colors.roll(3);    # red green red  (can repeat) 

Lists numify to their number of elements in numeric context

You want to check whether a number is divisible by any of a set of numbers:

say 42 %% <11 33 88 55 111 20325>; # OUTPUT: «True␤»

What? There's no single number 42 should be divisible by. However, that list has 6 elements, and 42 is divisible by 6. That's why the output is true. In this case, you should turn the List into a Junction:

say 42 %% <11 33 88 55 111 20325>.any;
# OUTPUT: «any(False, False, False, False, False, False)␤» 

which will clearly reveal the falsehood of the divisiveness of all the numbers in the list, which will be numified separately.

Arrays

Referencing the last element of an array

In some languages one could reference the last element of an array by asking for the "-1th" element of the array, e.g.:

my @array = qw{victor alice bob charlie eve};
say @array[-1];    # OUTPUT: «eve␤» 

In Raku it is not possible to use negative subscripts, however the same is achieved by actually using a function, namely *-1. Thus, accessing the last element of an array becomes:

my @array = qw{victor alice bob charlie eve};
say @array[*-1];   # OUTPUT: «eve␤» 

Yet another way is to utilize the array's tail method:

my @array = qw{victor alice bob charlie eve};
say @array.tail;      # OUTPUT: «eve␤» 
say @array.tail(2);   # OUTPUT: «(charlie eve)␤» 

Typed array parameters

Quite often new users will happen to write something like:

sub foo(Array @a) { ... }

...before they have gotten far enough in the documentation to realize that this is asking for an Array of Arrays. To say that @a should only accept Arrays, use instead:

sub foo(@a where Array) { ... }

It is also common to expect this to work, when it does not:

sub bar(Int @a) { 42.say };
bar([1, 2, 3]);             # expected Positional[Int] but got Array 

The problem here is that [1, 2, 3] is not an Array[Int], it is a plain old Array that just happens to have Ints in it. To get it to work, the argument must also be an Array[Int].

my Int @b = 1, 2, 3;
bar(@b);                    # OUTPUT: «42␤» 
bar(Array[Int].new(1, 2, 3));

This may seem inconvenient, but on the upside it moves the type-check on what is assigned to @b to where the assignment happens, rather than requiring every element to be checked on every call.

Using «» quoting when you don't need it

This trap can be seen in different varieties. Here are some of them:

my $x = ‘hello’;
my $y = ‘foo bar’;
 
my %h = $x => 42, $y => 99;
say %h«$x»;   # ← WRONG; assumption that $x has no whitespace 
say %h«$y»;   # ← WRONG; splits ‘foo bar’ by whitespace 
say %h«"$y"»; # ← KINDA OK; it works but there is no good reason to do that 
say %h{$y};   # ← RIGHT; this is what should be used 
 
run «touch $x»;        # ← WRONG; assumption that only one file will be created 
run «touch $y»;        # ← WRONG; will touch file ‘foo’ and ‘bar’ 
run «touch "$y"»;      # ← WRONG; better, but has a different issue if $y starts with - 
run «touch -- "$y"»;   # ← KINDA OK; it works but there is no good enough reason to do that 
run ‘touch’, ‘--’, $y; # ← RIGHT; explicit and *always* correct 
run <touch -->, $y;    # ← RIGHT; < > are OK, this is short and correct 

Basically, «» quoting is only safe to use if you remember to always quote your variables. The problem is that it inverts the default behavior to unsafe variant, so just by forgetting some quotes you are risking to introduce either a bug or maybe even a security hole. To stay on the safe side, refrain from using «».

Strings

Some problems that might arise when dealing with strings.

Quotes and interpolation

Interpolation in string literals can be too clever for your own good.

# "HTML tags" interpreted as associative indexing: 
"$foo<html></html>" eq
"$foo{'html'}{'/html'}"

# Parentheses interpreted as call with argument: 
"$foo(" ~ @args ~ ")" eq
"$foo(' ~ @args ~ ')"

You can avoid those problems using non-interpolating single quotes and switching to more liberal interpolation with \qq[] escape sequence:

my $a = 1;
say '\qq[$a]()$b()';
# OUTPUT: «1()$b()␤» 

Another alternative is to use Q:c quoter, and use code blocks {} for all interpolation:

my $a = 1;
say Q:c«{$a}()$b()»;
# OUTPUT: «1()$b()␤» 

Beware of variables used within qqx

Variables within qqx[] can introduce a security hole; the variable content can be set to well-crafted string and execute arbitrary code:

my $world = "there\";rm -rf /path/to/dir\"";
say qqx{echo "hello $world"};
# OUTPUT: «hello there␤» 

The above code will also delete /path/to/dir, you can avoid this problem by making sure the variable content does not have shell special characters, or use run and Proc::Async for better ways to execute external commands.

Strings are not iterable

There are methods that strings inherits from Any that work on iterables like lists. Iterators on strings contain one element that is the whole string. To use list-based methods like sort, reverse, you need to convert the string into a list first.

say "cba".sort;              # OUTPUT: «(cba)␤» 
say "cba".comb.sort.join;    # OUTPUT: «abc␤» 

.chars gets the number of graphemes, not Codepoints

In Raku, .chars returns the number of graphemes, or user visible characters. These graphemes could be made up of a letter plus an accent for example. If you need the number of codepoints, you should use .codes. If you need the number of bytes when encoded as UTF8, you should use .encode.bytes to encode the string as UTF8 and then get the number of bytes.

say "\c[LATIN SMALL LETTER J WITH CARON, COMBINING DOT BELOW]"; # OUTPUT: «ǰ̣» 
say 'ǰ̣'.codes;        # OUTPUT: «2» 
say 'ǰ̣'.chars;        # OUTPUT: «1» 
say 'ǰ̣'.encode.bytes; # OUTPUT: «4»

For more information on how strings work in Raku, see the Unicode page.

All text is normalized by default

Raku normalizes all text into Unicode NFC form (Normalization Form Canonical). Filenames are the only text not normalized by default. If you are expecting your strings to maintain a byte for byte representation as the original, you need to use UTF8-C8 when reading or writing to any filehandles.

Allomorphs generally follow numeric semantics

strings "0" is True, while Numeric is False. So what's the Bool value of allomorph <0>?

In general, allomorphs follow Numeric semantics, so the ones that numerically evaluate to zero are False:

say so   <0>; # OUTPUT: «False␤» 
say so <0e0>; # OUTPUT: «False␤» 
say so <0.0>; # OUTPUT: «False␤»

To force comparison being done for the Stringy part of the allomorph, use concatenation operator or the strings method to coerce the allomorph to strings, or use the .chars routine to test whether the allomorph has any length:

say so      ~<0>;     # OUTPUT: «True␤» 
say so       <0>.Str; # OUTPUT: «True␤» 
say so chars <0>;     # OUTPUT: «True␤»

Case-insensitive comparison of strings

In order to do case-insensitive comparison, you can use .fc (fold-case). The problem is that people tend to use .lc or .uc, and it does seem to work within the ASCII range, but fails on other characters. This is not just a Raku trap, the same applies to other languages.

say ‘groß’.lc eq ‘GROSS’.lc; # ← WRONG; False 
say ‘groß’.uc eq ‘GROSS’.uc; # ← WRONG; True, but that's just luck 
say ‘groß’.fc eq ‘GROSS’.fc; # ← RIGHT; True 

If you are working with regexes, then there is no need to use .fc and you can use :i (:ignorecase) adverb instead.

Pairs

Constants on the left-hand side of pair notation

Consider this code:

enum Animals <Dog Cat>;
my %h := :{ Dog => 42 };
say %h{Dog}; # OUTPUT: «(Any)␤» 

The :{ … } syntax is used to create object hashes. The intentions of someone who wrote that code were to create a hash with Enum objects as keys (and say %h{Dog} attempts to get a value using the Enum object to perform the lookup). However, that's not how pair notation works.

For example, in Dog => 42 the key will be a Str. That is, it doesn't matter if there is a constant, or an enumeration with the same name. The pair notation will always use the left-hand side as a string literal, as long as it looks like an identifier.

To avoid this, use (Dog) => 42 or ::Dog => 42.

Scalar values within Pair

When dealing with Scalar values, the Pair holds the container to the value. This means that it is possible to reflect changes to the Scalar value from outside the Pair:

my $v = 'value A';
my $pair = Pair.new( 'a', $v );
$pair.say;  # OUTPUT: a => value A 
 
$v = 'value B';
$pair.say; # OUTPUT: a => value B 

Use the method freeze to force the removal of the Scalar container from the Pair. For more details see the documentation about Pair.

Sets, bags and mixes

Sets, bags and mixes do not have a fixed order

When iterating over this kind of objects, an order is not defined.

my $set = <a b c>.Set;
.say for $set.list; # OUTPUT: «a => True␤c => True␤b => True␤» 
# OUTPUT: «a => True␤c => True␤b => True␤» 
# OUTPUT: «c => True␤b => True␤a => True␤» 

Every iteration might (and will) yield a different order, so you cannot trust a particular sequence of the elements of a set. If order does not matter, just use them that way. If it does, use sort

my $set = <a b c>.Set;
.say for $set.list.sort;  # OUTPUT: «a => True␤b => True␤c => True␤»

In general, sets, bags and mixes are unordered, so you should not depend on them having a particular order.

Operators

Some operators commonly shared among other languages were repurposed in Raku for other, more common, things:

Junctions

The ^, |, and & are not bitwise operators, they create Junction. The corresponding bitwise operators in Raku are: +^, +|, +& for integers and ?^, ?|, ?& for Bool.

Exclusive sequence operator

Lavish use of whitespace helps readability, but keep in mind infix operators cannot have any whitespace in them. One such operator is the sequence operator that excludes right point: ...^ (or its Unicode equivalent …^).

say 1... ^5; # OUTPUT: «(1 0 1 2 3 4)␤» 
say 1...^5;  # OUTPUT: «(1 2 3 4)␤»

If you place whitespace between the ellipsis (…) and the caret (^), it's no longer a single infix operator, but an infix inclusive sequence operator (…) and a prefix Range operator (^). Iterables are valid endpoints for the sequence operator, so the result you'll get might not be what you expected.

String ranges/Sequences

In some languages, using strings as range end points, considers the entire string when figuring out what the next string should be; loosely treating the strings as numbers in a large base. Here's the Perl version:

say join ", ", "az".."bc";
# OUTPUT: «az, ba, bb, bc␤» 

Such a range in Raku will produce a different result, where each letter will be ranged to a corresponding letter in the end point, producing more complex sequences:

say join ", ", "az".."bc";
#`{ OUTPUT: «
    az, ay, ax, aw, av, au, at, as, ar, aq, ap, ao, an, am, al, ak, aj, ai, ah,
    ag, af, ae, ad, ac, bz, by, bx, bw, bv, bu, bt, bs, br, bq, bp, bo, bn, bm,
    bl, bk, bj, bi, bh, bg, bf, be, bd, bc
␤»}

say join ", ", "r2".."t3";
# OUTPUT: «r2, r3, s2, s3, t2, t3␤» 

To achieve simpler behavior, similar to the Perl example above, use a sequence operator that calls .succ method on the starting string:

say join ", ", ("az", *.succ ... "bc");
# OUTPUT: «az, ba, bb, bc␤» 

Topicalizing operators

The smartmatch operator ~~ and andthen set the topic $_ to their left-hand-side. In conjunction with implicit method calls on the topic this can lead to surprising results.

my &method = { note $_; $_ };
$_ = 'object';
say .&method;
# OUTPUT: «object␤object␤» 
say 'topic' ~~ .&method;
# OUTPUT: «topic␤True␤» 

In many cases flipping the method call to the LHS will work.

my &method = { note $_; $_ };
$_ = 'object';
say .&method;
# OUTPUT: «object␤object␤» 
say .&method ~~ 'topic';
# OUTPUT: «object␤False␤» 

Fat arrow and constants

The fat arrow operator => will turn words on its left-hand side to Str without checking the scope for constants or \-sigiled variables. Use explicit scoping to get what you mean.

constant V = 'x';
my %h = V => 'oi‽', ::V => 42;
say %h.raku
# OUTPUT: «{:V("oi‽"), :x(42)}␤» 

Infix operator assignment

Infix operators, both built in and user defined, can be combined with the assignment operator as this addition example demonstrates:

my $x = 10;
$x += 20;
say $x;     # OUTPUT: «30␤»

For any given infix operator op, L op= R is equivalent to L = L op R (where L and R are the left and right arguments, respectively). This means that the following code may not behave as expected:

my @a = 1, 2, 3;
@a += 10;
say @a;  # OUTPUT: «[13]␤»

Coming from a language like C++, this might seem odd. It is important to bear in mind that += isn't defined as method on the left hand argument (here the @a array) but is simply shorthand for:

my @a = 1, 2, 3;
@a = @a + 10;
say @a;  # OUTPUT: «[13]␤»

Here @a is assigned the result of adding @a (which has three elements) and 10; 13 is therefore placed in @a.

Use the hyper form of the assignment operators instead:

my @a = 1, 2, 3;
@a »+=» 10;
say @a;  # OUTPUT: «[11 12 13]␤»

Regexes

$x vs <$x>, and $(code) vs <{code}>

Raku offers several constructs to generate regexes at runtime through interpolation (see their detailed description here). When a regex generated this way contains only literals, the above constructs behave (pairwise) identically, as if they are equivalent alternatives. As soon as the generated regex contains metacharacters, however, they behave differently, which may come as a confusing surprise.

The first two constructs that may easily be confused with each other are $variable and <$variable>:

my $variable = 'camelia';
say ‘I ♥ camelia’ ~~ /  $variable  /;   # OUTPUT: ｢camelia｣ 
say ‘I ♥ camelia’ ~~ / <$variable> /;   # OUTPUT: ｢camelia｣

Here they act the same because the value of $variable consists of literals. But when the variable is changed to comprise regex metacharacters the outputs become different:

my $variable = '#camelia';
say ‘I ♥ #camelia’ ~~ /  $variable  /;   # OUTPUT: ｢#camelia｣ 
say ‘I ♥ #camelia’ ~~ / <$variable> /;   # !! Error: malformed regex

What happens here is that the string #camelia contains the metacharacter #. In the context of a regex, this character should be quoted to match literally; without quoting, the # is parsed as the start of a comment that runs until the end of the line, which in turn causes the regex not to be terminated, and thus to be malformed.

Two other constructs that must similarly be distinguished from one another are $(code) and <{code}>. Like before, as long as the (stringified) return value of code comprises only literals, there is no distinction between the two:

my $variable = 'ailemac';
say ‘I ♥ camelia’ ~~ / $($variable.flip)   /;   # OUTPUT: ｢camelia｣ 
say ‘I ♥ camelia’ ~~ / <{$variable.flip}>  /;   # OUTPUT: ｢camelia｣

But when the return value is changed to comprise regex metacharacters, the outputs diverge:

my $variable = 'ailema.';
say ‘I ♥ camelia’ ~~ / $($variable.flip)   /;   # OUTPUT: Nil 
say ‘I ♥ camelia’ ~~ / <{$variable.flip}>  /;   # OUTPUT: ｢camelia｣

In this case the return value of the code is the string .amelia, which contains the metacharacter .. The above attempt by $(code) to match the dot literally fails; the attempt by <{code}> to match the dot as a regex wildcard succeeds. Hence the different outputs.

| vs ||: which branch will win

To match one of several possible alternatives, || or | will be used. But they are so different.

When there are multiple matching alternations, for those separated by ||, the first matching alternation wins; for those separated by |, which to win is decided by LTM strategy. See also: documentation on || and documentation on |.

For simple regexes just using || instead of | will get you familiar semantics, but if writing grammars then it's useful to learn about LTM and declarative prefixes and prefer |. And keep yourself away from using them in one regex. When you have to do that, add parentheses and ensure that you know how LTM strategy works to make the code do what you want.

The trap typically arises when you try to mix both | and || in the same regex:

say 42 ~~ / [  0 || 42 ] | 4/; # OUTPUT: «｢4｣␤» 
say 42 ~~ / [ 42 ||  0 ] | 4/; # OUTPUT: «｢42｣␤» 

The code above may seem like it is producing a wrong result, but the implementation is actually right.

$/ changes each time a regular expression is matched

Each time a regular expression is matched against something, the special variable $/ holding the result Match object is changed accordingly to the result of the match (that could also be Nil).

The $/ is changed without any regard to the scope the regular expression is matched within.

For further information and examples please see the related section in the Regular Expressions documentation.

<foo> vs. < foo>: named rules vs. quoted lists

Regexes can contain quoted lists; longest token matching is performed on the list's elements as if a | alternation had been specified (see here for further information).

Within a regex, the following are lists with a single item, 'foo':

say 'foo' ~~ /< foo >/;  # OUTPUT: «｢foo｣␤» 
say 'foo' ~~ /< foo>/;   # OUTPUT: «｢foo｣␤»

but this is a call to the named rule foo:

say 'foo' ~~ /<foo>/;
# OUTPUT: «No such method 'foo' for invocant of type 'Match'␤ in block <unit> at <unknown file> line 1␤» 

Be wary of the difference; if you intend to use a quoted list, ensure that whitespace follows the initial <.

Non-capturing, non-global matching in list context

Unlike Perl, non-capturing and non-global matching in list context doesn't produce any values:

if  'x' ~~ /./ { say 'yes' }  # OUTPUT: «yes␤» 
for 'x' ~~ /./ { say 'yes' }  # NO OUTPUT

This is because its 'list' slot (inherited from Capture class) doesn't get populated with the original Match object:

say ('x' ~~ /./).list  # OUTPUT: «()␤»

To achieve the desired result, use global matching, capturing parentheses or a list with a trailing comma:

for 'x' ~~ m:g/./ { say 'yes' }  # OUTPUT: «yes␤» 
for 'x' ~~ /(.)/  { say 'yes' }  # OUTPUT: «yes␤» 
for ('x' ~~ /./,) { say 'yes' }  # OUTPUT: «yes␤»

Common precedence mistakes

Adverbs and precedence

Adverbs do have a precedence that may not follow the order of operators that is displayed on your screen. If two operators of equal precedence are followed by an adverb it will pick the first operator it finds in the abstract syntax tree. Use parentheses to help Raku understand what you mean or use operators with looser precedence.

my %x = a => 42;
say !%x<b>:exists;            # dies with X::AdHoc 
say %x<b>:!exists;            # this works 
say !(%x<b>:exists);          # works too 
say not %x<b>:exists;         # works as well 
say True unless %x<b>:exists; # avoid negation altogether 

Ranges and precedence

The loose precedence of .. can lead to some errors. It is usually best to parenthesize ranges when you want to operate on the entire range.

1..3.say;    # OUTPUT: «3␤» (and warns about useless use of "..") 
(1..3).say;  # OUTPUT: «1..3␤» 

Loose Boolean operators

The precedence of and, or, etc. is looser than routine calls. This can have surprising results for calls to routines that would be operators or statements in other languages like return, last and many others.

sub f {
    return True and False;
    # this is actually 
    # (return True) and False; 
}
say f; # OUTPUT: «True␤» 

Exponentiation operator and prefix minus

say -1²;   # OUTPUT: «-1␤» 
say -1**2; # OUTPUT: «-1␤» 

When performing a regular mathematical calculation, the power takes precedence over the minus; so -1² can be written as -(1²). Raku matches these rules of mathematics and the precedence of ** operator is tighter than that of the prefix -. If you wish to raise a negative number to a power, use parentheses:

say (-1)²;   # OUTPUT: «1␤» 
say (-1)**2; # OUTPUT: «1␤» 

Method operator calls and prefix minus

Prefix minus binds looser than dotty method op calls. The prefix minus will be applied to the return value from the method. To ensure the minus gets passed as part of the argument, enclose in parenthesis.

say  -1.abs;  # OUTPUT: «-1␤» 
say (-1).abs; # OUTPUT: «1␤» 

Subroutine and method calls

Subroutine and method calls can be made using one of two forms:

foo(...); # function call form, where ... represent the required arguments 
foo ...;  # list op form, where ... represent the required arguments 

The function call form can cause problems for the unwary when whitespace is added after the function or method name and before the opening parenthesis.

First we consider functions with zero or one parameter:

sub foo() { say 'no arg' }
sub bar($a) { say "one arg: $a" }

Then execute each with and without a space after the name:

foo();    # okay: no arg 
foo ();   # FAIL: Too many positionals passed; expected 0 arguments but got 1 
bar($a);  # okay: one arg: 1 
bar ($a); # okay: one arg: 1 

Now declare a function of two parameters:

sub foo($a, $b) { say "two args: $a, $b" }

Execute it with and without the space after the name:

foo($a, $b);  # okay: two args: 1, 2 
foo ($a, $b); # FAIL: Too few positionals passed; expected 2 arguments but got 1 

The lesson is: "be careful with spaces following sub and method names when using the function call format." As a general rule, good practice might be to avoid the space after a function name when using the function call format.

Note that there are clever ways to eliminate the error with the function call format and the space, but that is bordering on hackery and will not be mentioned here. For more information, consult Functions.

Finally, note that, currently, when declaring the functions whitespace may be used between a function or method name and the parentheses surrounding the parameter list without problems.

Named parameters

Many built-in subroutines and method calls accept named parameters and your own code may accept them as well, but be sure the arguments you pass when calling your routines are actually named parameters:

sub foo($a, :$b) { ... }
foo(1, 'b' => 2); # FAIL: Too many positionals passed; expected 1 argument but got 2 

What happened? That second argument is not a named parameter argument, but a Pair passed as a positional argument. If you want a named parameter it has to look like a name to Perl:

foo(1, b => 2); # okay 
foo(1, :b(2));  # okay 
foo(1, :b<it>); # okay 
 
my $b = 2;
foo(1, :b($b)); # okay, but redundant 
foo(1, :$b);    # okay 
 
# Or even... 
my %arg = 'b' => 2;
foo(1, |%arg);  # okay too 

That last one may be confusing, but since it uses the | prefix on a Hash, which is a special compiler construct indicating you want to use the contents of the variable as arguments, which for hashes means to treat them as named arguments.

If you really do want to pass them as pairs you should use a List or Capture instead:

my $list = ('b' => 2),; # this is a List containing a single Pair 
foo(|$list, :$b);       # okay: we passed the pair 'b' => 2 to the first argument 
foo(1, |$list);         # FAIL: Too many positionals passed; expected 1 argument but got 2 
foo(1, |$list.Capture); # OK: .Capture call converts all Pair objects to named args in a Capture 

my $cap = \('b' => 2); # a Capture with a single positional value 
foo(|$cap, :$b); # okay: we passed the pair 'b' => 2 to the first argument 
foo(1, |$cap);   # FAIL: Too many positionals passed; expected 1 argument but got 2 

A Capture is usually the best option for this as it works exactly like the usual capturing of routine arguments during a regular call.

The nice thing about the distinction here is that it gives the developer the option of passing pairs as either named or positional arguments, which can be handy in various instances.

Argument count limit

While it is typically unnoticeable, there is a backend-dependent argument count limit. Any code that does flattening of arbitrarily sized arrays into arguments won't work if there are too many elements.

my @a = 1 xx 9999;
my @b;
@b.push: |@a;
say @b.elems # OUTPUT: «9999␤» 

my @a = 1 xx 999999;
my @b;
@b.push: |@a; # OUTPUT: «Too many arguments in flattening array.␤  in block <unit> at <tmp> line 1␤␤» 

Avoid this trap by rewriting the code so that there is no flattening. In the example above, you can replace push with append. This way, no flattening is required because the array can be passed as is.

my @a = 1 xx 999999;
my @b;
@b.append: @a;
say @b.elems # OUTPUT: «999999␤» 

Phasers and implicit return

sub returns-ret () {
    CATCH {
        default {}
    }
    "ret";
}
 
sub doesn't-return-ret () {
    "ret";
    CATCH {
        default {}
    }
}
 
say returns-ret;        # OUTPUT: «ret» 
say doesn't-return-ret;
# BAD: outputs «Nil» and a warning «Useless use of constant string "ret" in sink context (line 13)» 

Code for returns-ret and doesn't-return-ret might look exactly the same, since in principle it does not matter where the CATCH block goes. However, a block is an object and the last object in a sub will be returned, so the doesn't-return-ret will return Nil, and, besides, since "ret" will be now in sink context, it will issue a warning. In case you want to place phasers last for conventional reasons, use the explicit form of return.

sub explicitly-return-ret () {
    return "ret";
    CATCH {
        default {}
    }
}

LEAVE needs explicit return from a sub to run

As the documentation for the LEAVE phaser indicates LEAVE runs when a block is exited, "... except when the program exits abruptly". That is, unlike END, it's going to be invoked only if the block actually returns in an orderly way. This is why:

sub a() { LEAVE say "left"; exit 1 }; # No output, it will simply exit 

will not run the LEAVE code, since technically it's not returning. On the other hand, END is a program execution phaser and will run no matter what

sub a() {
    END say "Begone"; exit 1
}; a; # OUTPUT: «Begone␤»