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YSH Expression Language

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

Table of Contents

Keywords

const

Initializes a constant name to the Oil expression on the right.

const c = 'mystr'        # equivalent to readonly c=mystr
const pat = / digit+ /   # an eggex, with no shell equivalent

It's either a global constant or scoped to the current function.

var

Initializes a name to the Oil expression on the right.

var s = 'mystr'        # equivalent to declare s=mystr
var pat = / digit+ /   # an eggex, with no shell equivalent

It's either global or scoped to the current function.

setvar

At the top-level, setvar creates or mutates a variable.

Inside a proc, it mutates a local variable declared with var.

setglobal

Creates or mutates a global variable.

setref

Mutates a variable through a named reference. See examples in doc/variables.md.

Literals

bool-literal

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

true   false   null

Note that the empty string is a good "special" value in some cases. The null value can't be interpolated into words.

int-literal

var myint = 42
var myfloat = 3.14
var float2 = 1e100

rune-literal

#'a'   #'_'   \n   \\   \u{3bc}

str-literal

Oil strings appear in expression contexts, and look like shell strings:

var s = 'foo'
var double = "hello $world and $(hostname)"

However, strings with backslashes are forced to specify whether they're raw strings or C-style strings:

var s = 'line\n'    # parse error: ambiguous

var s = $'line\n'   # C-style string

var s = r'[a-z]\n'  # raw strings are useful for regexes (not eggexes)

var unicode = 'mu = \u{3bc}'

list-literal

Lists have a Python-like syntax:

var mylist = ['one', 'two', 3]

And a shell-like syntax:

var list2 = %| one two |

The shell-like syntax accepts the same syntax that a command can:

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

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

dict-literal

{name: 'value'}

block-literal

var myblock = ^(echo $PWD)

expr-lit

var myexpr = ^[1 + 2*3]

Operators

concat

var s = 's'
var concat1 = s ++ '_suffix'
var concat2 = "${s}_suffix"  # similar

var c = :| one two |
var concat3 = c ++ :| three 4 |
var concat4 = :| @c three 4 |

var mydict = {a: 1, b: 2}
var otherdict = {a: 10, c: 20}
var concat5 = mydict ++ otherdict

ysh-compare

a == b        # Python-like equality, no type conversion
3 ~== 3.0     # True, type conversion
3 ~== '3'     # True, type conversion
3 ~== '3.0'   # True, type conversion

ysh-logical

not  and  or

Note that these are distinct from ! && ||.

ysh-arith

+  -  *  /   //   %   **

ysh-bitwise

~  &  |  ^

ysh-ternary

Like Python:

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

ysh-index

Like Python:

myarray[3]
mystr[3]

TODO: Does string indexing give you an integer back?

ysh-slice

Like Python:

myarray[1 : -1]
mystr[1 : -1]

func-call

Like Python:

f(x, y)

match-ops

~   !~   ~~   !~~