Oils Reference — Chapter YSH Expression Language

This chapter describes the YSH expression language, which includes Egg Expressions.

In This Chapter
Assignment
assign
aug-assign
Literals
atom-literal
int-literal
float-lit
ysh-string
triple-quoted
str-template
list-literal
dict-literal
range
block-expr
expr-literal
Operators
op-precedence
concat ++
ysh-equals
ysh-in
ysh-compare
ysh-logical
ysh-arith
ysh-bitwise
ysh-ternary
ysh-index
ysh-attr
ysh-slice
func-call
thin-arrow
fat-arrow
match-ops
Eggex
re-literal
re-primitive
class-literal
named-class
re-repeat
re-compound
re-capture
re-splice
re-flags
re-multiline

Assignment

assign

The = operator is used with assignment keywords:

var x = 42
setvar x = 43

const y = 'k'

setglobal z = 'g'

aug-assign

The augmented assignment operators are:

+=   -=   *=   /=   **=   //=   %=
&=   |=   ^=   <<=   >>=

They are used with setvar and setglobal. For example:

setvar x += 2

is the same as:

setvar x = x + 2

Likewise, these are the same:

setglobal a[i] -= 1

setglobal a[i] = a[i] - 1

Literals

atom-literal

YSH uses JavaScript-like spellings for these three "atoms":

null           # type Null
true   false   # type Bool

Note: to signify "no value", you may sometimes use an empty string '', instead of null.

int-literal

Examples of integer literals:

var decimal = 42
var big = 42_000

var hex = 0x0010_ffff

var octal = 0o755

var binary = 0b0001_0000

float-lit

Examples of float literals:

var myfloat = 3.14

var f2 = -1.5e-100

ysh-string

YSH has single and double-quoted strings borrowed from Bourne shell, and C-style strings borrowed from J8 Notation.

Double quoted strings respect $ interpolation:

var dq = "hello $world and $(hostname)"

You can add a $ before the left quote to be explicit: $"x is $x" rather than "x is $x".

Single quoted strings may be raw:

var s = r'line\n'      # raw string means \n is literal, NOT a newline

Or J8 strings with backslash escapes:

var s = u'line\n \u{3bc}'        # unicode string means \n is a newline
var s = b'line\n \u{3bc} \yff'   # same thing, but also allows bytes

Both u'' and b'' strings evaluate to the single Str type. The difference is that b'' strings allow the \yff byte escape.

Notes

There's no way to express a single quote in raw strings. Use one of the other forms instead:

var sq = "single quote: ' "
var sq = u'single quote: \' '

Sometimes you can omit the r, e.g. where there are no backslashes and thus no ambiguity:

echo 'foo'
echo r'foo'  # same thing

The u'' and b'' strings are called J8 strings because the syntax in YSH code matches JSON-like data.

var strU = u'mu = \u{3bc}'  # J8 string with escapes
var strB = b'bytes \yff'    # J8 string that can express byte strings

More examples:

var myRaw = r'[a-z]\n'      # raw strings can be used for regexes (not
                            # eggexes)

triple-quoted

Triple-quoted string literals have leading whitespace stripped on each line. They come in the same variants:

var dq = """
    hello $world and $(hostname)
    no leading whitespace
    """

var myRaw = r'''
    raw string
    no leading whitespace
    '''

var strU = u'''
    string that happens to be unicode \u{3bc}
    no leading whitespace
    '''

var strB = b'''
    string that happens to be bytes \u{3bc} \yff
    no leading whitespace
    '''

Again, you can omit the r prefix if there's no backslash, because it's not ambiguous:

var myRaw = '''
    raw string
    no leading whitespace
    '''

str-template

String templates use the same syntax as double-quoted strings:

var mytemplate = ^"name = $name, age = $age"

Related topics:

list-literal

Lists have a Python-like syntax:

var mylist = ['one', 'two', [42, 43]]

And a shell-like syntax:

var list2 = :| one two |

The shell-like syntax accepts the same syntax as a simple command:

ls $mystr @ARGV *.py {foo,bar}@example.com

# Rather than executing ls, evaluate words into a List
var cmd = :| ls $mystr @ARGV *.py {foo,bar}@example.com |

dict-literal

Dicts look like JavaScript.

var d = {
  key1: 'value',  # key can be unquoted if it looks like a var name
  'key2': 42,     # or quote it

  ['key2' ++ suffix]: 43,   # bracketed expression
}

Omitting a value means that the corresponding key takes the value of a var of the same name:

ysh$ var x = 42
ysh$ var y = 43

ysh$ var d = {x, y}  # values omitted
ysh$ = d
(Dict)  {x: 42, y: 43}

range

A range is a sequence of numbers that can be iterated over:

for i in (0 .. 3) {
  echo $i
}
=> 0
=> 1
=> 2

As with slices, the last number isn't included. To iterate from 1 to n, you can use this idiom:

for i in (1 .. n+1) {
  echo $i
}

block-expr

In YSH expressions, we use ^() to create a Command object:

var myblock = ^(echo $PWD; ls *.txt)

It's more common for Command objects to be created with block arguments, which are not expressions:

cd /tmp {
  echo $PWD
  ls *.txt
}

expr-literal

An expression literal is an object that holds an unevaluated expression:

var myexpr = ^[1 + 2*3]

Operators

op-precedence

YSH operator precedence is identical to Python's operator precedence.

New operators:

concat ++

The concatenation operator works on Str objects:

ysh$ var s = 'hello'
ysh$ var t = s ++ ' world'

ysh$ = t
(Str)   "hello world"

and List objects:

ysh$ var L = ['one', 'two']
ysh$ var M = L ++ ['three', '4']

ysh$ = M
(List)   ["one", "two", "three", "4"]

String interpolation can be nicer than ++:

var t2 = "${s} world"  # same as t

Likewise, splicing lists can be nicer:

var M2 = :| @L three 4 |  # same as M

ysh-equals

YSH has strict equality:

a === b       # Python-like, without type conversion
a !== b       # negated

And type converting equality:

'3' ~== 3     # True, type conversion

The ~== operator expects a string as the left operand.

Note that:

You may want to use explicit int() and float() to convert numbers, and then compare them.

Compare objects for identity with is:

ysh$ var d = {}    
ysh$ var e = d

ysh$ = d is d
(Bool)   true

ysh$ = d is {other: 'dict'}
(Bool)   false

To negate is, use is not (like Python:

ysh$ d is not {other: 'dict'}
(Bool)   true

ysh-in

The in operator tests if a key is in a dictionary:

var d = {k: 42}
if ('k' in d) {
  echo yes
}  # => yes

Unlike Python, in doesn't work on Str and List instances. This because those operations take linear time rather than constant time (O(n) rather than O(1)).

TODO: Use includes() / contains() methods instead.

ysh-compare

The comparison operators apply to integers or floats:

4 < 4   # => false
4 <= 4  # => true

5.0 > 5.0   # => false
5.0 >= 5.0  # => true

Example in context:

if (x < 0) {
  echo 'x is negative'
}

ysh-logical

The logical operators take boolean operands, and are spelled like Python:

not
and  or

Note that they are distinct from ! && ||, which are part of the command language.

ysh-arith

YSH supports most of the arithmetic operators from Python. Notably, / and % differ from Python as they round toward zero, not negative infinity.

Use + - * for Int or Float addition, subtraction and multiplication. If any of the operands are Floats, then the output will also be a Float.

Use / and // for Float division and Int division, respectively. / will always result in a Float, meanwhile // will always result in an Int.

= 1 / 2   # => (Float) 0.5
= 1 // 2  # => (Int) 0

Use % to compute the remainder of integer division. The left operand must be an Int and the right a positive Int.

= 1 % 2   # -> (Int) 1
= -4 % 2  # -> (Int) 0

Use ** for exponentiation. The left operand must be an Int and the right a positive Int.

All arithmetic operators may coerce either of their operands from strings to a number, provided those strings are formatted as numbers.

= 10 + '1'  # => (Int) 11

Operators like + - * / will coerce strings to either an Int or Float. However, operators like // ** % and bit shifts will coerce strings only to an Int.

= '1.14' + '2'  # => (Float) 3.14
= '1.14' % '2'  # Type Error: Left operand is a Str

ysh-bitwise

Bitwise operators are like Python and C:

~        # unary complement

&  |  ^  # binary and, or, xor

>>  <<   # bit shift

ysh-ternary

The ternary operator is borrowed from Python:

display = 'yes' if len(s) else 'empty'

ysh-index

Str objects can be indexed by byte:

ysh$ var s = 'cat'
ysh$ = mystr[1]
(Str)   'a'  

ysh$ = mystr[-1]  # index from the end
(Str)   't'

List objects:

ysh$ var mylist = [1, 2, 3]
ysh$ = mylist[2]
(Int)  3

Dict objects are indexed by string key:

ysh$ var mydict = {'key': 42}
ysh$ = mydict['key']
(Int)  42

ysh-attr

The expression mydict.key is short for mydict['key'].

(Like JavaScript, but unlike Python.)

ysh-slice

Slicing gives you a subsequence of a Str or List, like Python.

Negative indices are relative to the end.

func-call

A function call expression looks like Python:

ysh$ = f('s', 't', named=42)

A semicolon ; can be used after positional args and before named args, but isn't always required:

ysh$ = f('s', 't'; named=42)

In these cases, the ; is necessary:

ysh$ = f(...args; ...kwargs)

ysh$ = f(42, 43; ...kwargs)

thin-arrow

The thin arrow is for mutating methods:

var mylist = ['bar']
call mylist->pop()

fat-arrow

The fat arrow is for transforming methods:

if (s => startsWith('prefix')) {
  echo 'yes'
}

If the method lookup on s fails, it looks for free functions. This means it can be used for "chaining" transformations:

var x = myFunc() => list() => join()

match-ops

YSH has four pattern matching operators: ~ !~ ~~ !~~.

Does string match an eggex?

var filename = 'x42.py'
if (filename ~ / d+ /) {
  echo 'number'
}

Does a string match a POSIX regular expression (ERE syntax)?

if (filename ~ '[[:digit:]]+') {
  echo 'number'
}

Negate the result with the !~ operator:

if (filename !~ /space/ ) {
  echo 'no space'
}

if (filename !~ '[[:space:]]' ) {
  echo 'no space'
}

Does a string match a glob?

if (filename ~~ '*.py') {
  echo 'Python'
}

if (filename !~~ '*.py') {
  echo 'not Python'
}

Take care not to confuse glob patterns and regular expressions.

Eggex

re-literal

An eggex literal looks like this:

/ expression ; flags ; translation preference /

The flags and translation preference are both optional.

Examples:

var pat = / d+ /  # => [[:digit:]]+

You can specify flags passed to libc regcomp():

var pat = / d+ ; reg_icase reg_newline /

You can specify a translation preference after a second semi-colon:

var pat = / d+ ; ; ERE /

Right now the translation preference does nothing. It could be used to translate eggex to PCRE or Python syntax.

re-primitive

There are two kinds of eggex primitives.

"Zero-width assertions" match a position rather than a character:

%start           # translates to ^
%end             # translates to $

Literal characters appear within single quotes:

'oh *really*'    # translates to regex-escaped string

Double-quoted strings are not eggex primitives. Instead, you can use splicing of strings:

var dq = "hi $name"    
var eggex = / @dq /

class-literal

An eggex character class literal specifies a set. It can have individual characters and ranges:

[ 'x' 'y' 'z' a-f A-F 0-9 ]  # 3 chars, 3 ranges

Omit quotes on ASCII characters:

[ x y z ]  # avoid typing 'x' 'y' 'z'

Sets of characters can be written as strings

[ 'xyz' ]  # any of 3 chars, not a sequence of 3 chars

Backslash escapes are respected:

[ \\ \' \" \0 ]
[ \xFF \u0100 ]

Splicing:

[ @str_var ]

Negation always uses !

![ a-f A-F 'xyz' @str_var ]

named-class

Perl-like shortcuts for sets of characters:

[ dot ]    # => .
[ digit ]  # => [[:digit:]]
[ space ]  # => [[:space:]]
[ word ]   # => [[:alpha:]][[:digit:]]_

Abbreviations:

[ d s w ]  # Same as [ digit space word ]

Valid POSIX classes:

alnum   cntrl   lower   space
alpha   digit   print   upper
blank   graph   punct   xdigit

Negated:

!digit   !space   !word
!d   !s   !w
!alnum  # etc.

re-repeat

Eggex repetition looks like POSIX syntax:

/ 'a'? /      # zero or one
/ 'a'* /      # zero or more
/ 'a'+ /      # one or more

Counted repetitions:

/ 'a'{3} /    # exactly 3 repetitions
/ 'a'{2,4} /  # between 2 to 4 repetitions

re-compound

Sequence expressions with a space:

/ word digit digit /   # Matches 3 characters in sequence
                       # Examples: a42, b51

(Compare / [ word digit ] /, which is a set matching 1 character.)

Alternation with |:

/ word | digit /       # Matches 'a' OR '9', for example

Grouping with parentheses:

/ (word digit) | \\ /  # Matches a9 or \

re-capture

To retrieve a substring of a string that matches an Eggex, use a "capture group" like <capture ...>.

Here's an eggex with a positional capture:

var pat = / 'hi ' <capture d+> /  # access with _group(1)
                                  # or Match => _group(1)

Captures can be named:

<capture d+ as month>       # access with _group('month')
                            # or Match => group('month')

Captures can also have a type conversion func:

<capture d+ : int>          # _group(1) returns Int

<capture d+ as month: int>  # _group('month') returns Int

Related docs and help topics:

re-splice

To build an eggex out of smaller expressions, you can splice eggexes together:

var D = / [0-9][0-9] /
var time = / @D ':' @D /  # [0-9][0-9]:[0-9][0-9]

If the variable begins with a capital letter, you can omit @:

var ip = / D ':' D /

You can also splice a string:

var greeting = 'hi'
var pat = / @greeting ' world' /  # hi world

Splicing is not string concatenation; it works on eggex subtrees.

re-flags

Valid ERE flags, which are passed to libc's regcomp():

See man regcomp.

re-multiline

Multi-line eggexes aren't yet implemented. Splicing makes it less necessary:

var Name  = / <capture [a-z]+ as name> /
var Num   = / <capture d+ as num> /
var Space = / <capture s+ as space> /

# For variables named like CapWords, splicing @Name doesn't require @
var lexer = / Name | Num | Space /
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