| 1 | ---
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| 2 | default_highlighter: oils-sh
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| 3 | ---
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| 4 |
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| 5 | Command vs. Expression Mode
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| 6 | ===========================
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| 7 |
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| 8 | [YSH][] extends the shell **command** language with a Python-like
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| 9 | **expression** language.
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| 10 |
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| 11 | Commands and expressions each have a **lexer mode**, which is an essential
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| 12 | [syntactic concept](syntactic-concepts.html) in YSH.
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| 13 |
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| 14 | This doc lists the places where [YSH][] switches between modes.
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| 15 |
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| 16 | [YSH]: $xref
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| 17 |
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| 18 | <div id="toc">
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| 19 | </div>
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| 20 |
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| 21 | ## Summary
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| 22 |
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| 23 | A main difference is whether you write strings like `unquoted` or `'quoted'`,
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| 24 | and whether you write variables like `$dollar` or `unquoted`:
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| 25 |
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| 26 | <style>
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| 27 | thead { text-align: left; }
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| 28 | table {
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| 29 | width: 100%;
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| 30 | margin-left: 2em; /* match */
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| 31 | }
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| 32 | </style>
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| 33 |
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| 34 | <table>
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| 35 |
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| 36 | - thead
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| 37 | - Description
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| 38 | - Lexing Mode
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| 39 | - String
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| 40 | - Variable
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| 41 | - Example
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| 42 | - tr
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| 43 | - Shell-Like
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| 44 | - Command
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| 45 | - `unquoted`
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| 46 | - `$dollar`
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| 47 | - ```
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| 48 | ls foo/bar $myvar
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| 49 | ```
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| 50 | - tr
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| 51 | - Python-like
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| 52 | - Expression
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| 53 | - `'quoted'`
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| 54 | - `unquoted`
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| 55 | - ```
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| 56 | var s = myfunc('str', myvar)
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| 57 | ```
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| 58 |
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| 59 | </table>
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| 60 |
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| 61 | More examples:
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| 62 |
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| 63 | ls foo/bar # foo and bar are strings - command
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| 64 | var x = foo / bar # foo and bar are the names of variables - expression
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| 65 |
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| 66 | And:
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| 67 |
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| 68 | echo $filename.py # $filename is a var - command
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| 69 | var x = filename ++ '.py' # filename is a var - expression
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| 70 |
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| 71 | <!--
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| 72 | Shell has a similar difference:
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| 73 |
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| 74 | ls foo/bar # foo and bar are strings
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| 75 | a=$(( foo/bar )) # foo and bar are the names of variables
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| 76 | -->
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| 77 |
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| 78 |
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| 79 | ## From Command Mode to Expression Mode
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| 80 |
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| 81 | ### RHS of Assignments
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| 82 |
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| 83 | Everything after `=` is parsed in expression mode:
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| 84 |
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| 85 | var x = 42 + f(x) # RHS of var/setvar
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| 86 | setvar x += g(y)
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| 87 |
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| 88 | setvar x = obj.method()
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| 89 |
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| 90 | This includes *bare assignments* in Hay blocks:
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| 91 |
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| 92 | Rule {
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| 93 | x = 42 + a[i]
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| 94 | }
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| 95 |
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| 96 | ### `=` and `call` keywords
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| 97 |
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| 98 | Likewise, everything after `=` or `call` is in expression mode:
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| 99 |
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| 100 | = 42 + f(x)
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| 101 |
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| 102 | Throw away the value:
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| 103 |
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| 104 | call mylist->append(x)
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| 105 |
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| 106 | ### YSH `for while if case`:
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| 107 |
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| 108 | Expressions are surrounded by `( )`:
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| 109 |
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| 110 | for k, v in (mydict) {
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| 111 | echo "$k $v"
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| 112 | }
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| 113 |
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| 114 | while (x > 0) {
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| 115 | setvar x -= 1
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| 116 | }
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| 117 |
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| 118 | if (x > 0) {
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| 119 | echo 'positive'
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| 120 | }
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| 121 |
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| 122 | case (len(x)) {
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| 123 | (1) { echo one }
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| 124 | (2) { echo two }
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| 125 | (else) { echo other }
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| 126 | }
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| 127 |
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| 128 | ### Expression Sub and Splice
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| 129 |
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| 130 | The `$[]` construct converts an expression to a string:
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| 131 |
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| 132 | echo $[42 + a[i]]
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| 133 |
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| 134 | The `@[]` construct converts a list to an array of strings:
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| 135 |
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| 136 | echo @[arrayfunc('three', 'four', f(x))]
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| 137 |
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| 138 | ### Typed Arguments to Procs
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| 139 |
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| 140 | Typed arguments are surrounded by `( )`:
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| 141 |
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| 142 | json write (['three', 'four'])
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| 143 | # =>
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| 144 | [ "three", "four" ]
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| 145 |
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| 146 | Lazy arguments:
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| 147 |
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| 148 | assert [42 === x]
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| 149 |
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| 150 | ### Proc and Func Parameter Lists
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| 151 |
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| 152 | Parameters aren't expressions, but they're parsed with the same lexer:
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| 153 |
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| 154 | proc p (x, y) { # what's between () is in expression mode
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| 155 | echo "$x $y" # back to command mode
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| 156 | }
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| 157 |
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| 158 | func f(x) {
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| 159 | return (x)
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| 160 | }
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| 161 |
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| 162 | ## From Expression Mode to Command Mode
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| 163 |
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| 164 | ### Array Literals
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| 165 |
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| 166 | var myarray = :| /tmp/foo ${var} $(echo hi) @myarray |
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| 167 |
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| 168 | ### Command Sub, Command Literals
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| 169 |
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| 170 | Everything in between sigil pairs is in command mode:
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| 171 |
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| 172 | var x = $(hostname | tr a-z A-Z)
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| 173 |
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| 174 | var y = @(seq 3) # Split command sub
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| 175 |
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| 176 | This is a command literal:
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| 177 |
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| 178 | var b = ^(echo $PWD)
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| 179 |
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| 180 | ## More Examples
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| 181 |
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| 182 | ### How Are Glob Patterns Written in Each Mode?
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| 183 |
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| 184 | No:
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| 185 |
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| 186 | echo '*.py' # a literal string, not a glob
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| 187 |
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| 188 | echo @[glob(*.py)] # syntax error, * is an operator in
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| 189 | # expression mode
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| 190 |
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| 191 | var x = myfunc(*.py) # ditto, syntax error
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| 192 |
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| 193 | Yes:
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| 194 |
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| 195 | echo *.py # expanded as a glob
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| 196 |
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| 197 | echo @[glob('*.py')] # A literal string passed to the builtin
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| 198 | # glob function
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| 199 |
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| 200 | var x = f('*.py') # Just a string
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| 201 |
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| 202 | var x = f(glob('*.py')) # Now it's expanded
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| 203 |
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| 204 | Another way to say this is that YSH works like Python:
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| 205 |
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| 206 | ```python
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| 207 | from glob import glob
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| 208 | glob('*.py') # this is a glob
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| 209 | os.listdir('*.py') # no glob because it's not how listdir() works
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| 210 | ```
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| 211 |
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| 212 | Also note that YSH has a builtin operator that uses glob aka `fnmatch()`
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| 213 | syntax:
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| 214 |
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| 215 | if (x ~~ '*.py') {
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| 216 | echo 'Python'
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| 217 | }
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| 218 |
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