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 | This is an essential [syntactic concept](syntactic-concepts.html) in YSH.
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9 |
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10 | YSH extends the shell **command** language with a Python-like **expression**
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11 | language.
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12 |
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13 | To implement that, the lexer enters "expression mode".
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14 |
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15 | The key difference is that when lexing commands, `unquoted` is a string, while
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16 | `$dollar` is a variable:
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17 |
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18 | ls /bin/str $myvar
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19 |
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20 | On the other hand, when lexing expressions, `'quoted'` is a string, while
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21 | `unquoted` is a variable:
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22 |
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23 | var s = myfunc('str', myvar)
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24 |
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25 | This doc lists the places where we switch modes.
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26 |
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27 | <div id="toc">
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28 | </div>
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29 |
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30 | ## From Command Mode to Expression Mode
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31 |
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32 | ### RHS of Assignments
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33 |
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34 | Everything after `=` is parsed in expression mode:
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35 |
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36 | var x = 42 + f(x) # RHS of var/setvar
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37 | setvar x += g(y)
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38 |
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39 | setvar x = obj.method()
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40 |
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41 | This includes *bare assignments* in Hay blocks:
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42 |
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43 | Rule {
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44 | x = 42 + a[i]
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45 | }
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46 |
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47 | ### `=` and `call` keywords
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48 |
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49 | Likewise, everything after `=` or `::` is in expression mode:
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50 |
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51 | = 42 + f(x)
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52 |
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53 | Throw away the value:
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54 |
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55 | call mylist->append(x)
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56 |
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57 | ### YSH `for while if case`:
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58 |
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59 | Expressions are surrounded by `( )`:
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60 |
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61 | for k, v in (mydict) {
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62 | echo "$k $v"
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63 | }
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64 |
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65 | while (x > 0) {
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66 | setvar x -= 1
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67 | }
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68 |
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69 | if (x > 0) {
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70 | echo 'positive'
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71 | }
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72 |
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73 | case (len(x)) {
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74 | (1) { echo one }
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75 | (2) { echo two }
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76 | (else) { echo other }
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77 | }
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78 |
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79 | ### Expression Sub and Splice
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80 |
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81 | The `$[]` construct converts an expression to a string:
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82 |
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83 | echo $[42 + a[i]]
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84 |
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85 | The `@[]` construct converts a list to an array of strings:
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86 |
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87 | echo @[arrayfunc('three', 'four', f(x))]
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88 |
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89 | ### Typed Arguments to Procs
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90 |
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91 | Typed arguments are surrounded by `( )`:
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92 |
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93 | json write (['three', 'four'])
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94 | # =>
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95 | [ "three", "four" ]
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96 |
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97 | Lazy arguments:
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98 |
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99 | assert [42 === x]
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100 |
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101 | ### Proc and Func Parameter Lists
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102 |
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103 | Parameters aren't expressions, but they're parsed with the same lexer:
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104 |
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105 | proc p(x, y) { # what's between () is in expression mode
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106 | echo "$x $y" # back to command mode
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107 | }
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108 |
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109 | func f(x) {
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110 | return (x)
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111 | }
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112 |
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113 | ## From Expression Mode to Command Mode
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114 |
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115 | ### Array Literals
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116 |
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117 | var myarray = :| /tmp/foo ${var} $(echo hi) @myarray |
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118 |
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119 | ### Command Sub, Command Literals
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120 |
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121 | Everything in between sigil pairs is in command mode:
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122 |
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123 | var x = $(hostname | tr a-z A-Z)
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124 |
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125 | var y = @(seq 3) # Split command sub
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126 |
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127 | This is a command literal:
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128 |
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129 | var b = ^(echo $PWD)
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130 |
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131 | ## Examples
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132 |
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133 | ### How Are Glob Patterns Written in Each Mode?
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134 |
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135 | No:
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136 |
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137 | echo '*.py' # a literal string, not a glob
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138 |
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139 | echo @[glob(*.py)] # syntax error, * is an operator in
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140 | # expression mode
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141 |
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142 | var x = myfunc(*.py) # ditto, syntax error
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143 |
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144 | Yes:
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145 |
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146 | echo *.py # expanded as a glob
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147 |
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148 | echo @[glob('*.py')] # A literal string passed to the builtin
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149 | # glob function
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150 |
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151 | var x = f('*.py') # Just a string
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152 |
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153 | var x = f(glob('*.py')) # Now it's expanded
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154 |
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155 | Another way to say this is that YSH works like Python:
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156 |
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157 | ```python
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158 | from glob import glob
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159 | glob('*.py') # this is a glob
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160 | os.listdir('*.py') # no glob because it's not how listdir() works
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161 | ```
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162 |
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163 | Also note that YSH has a builtin operator that uses glob aka `fnmatch()`
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164 | syntax:
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165 |
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166 | if (x ~~ '*.py') {
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167 | echo 'Python'
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168 | }
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169 |
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170 |
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171 | ## vim: sw=2
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