Command vs. Expression Mode

This is an essential syntactic concept in Oil.

Oil is an extension of the shell language, which consists of commands. The most important addition is a Python-like expression language. To implement that, the lexer enters "expression mode".

Here's the key difference:

In command mode, unquoted is a string, while $dollar is a variable:

ls /bin/str $myvar

In expression mode: 'quoted' is a string, while unquoted is a variable:

var s = myfunc('str', myvar)
Table of Contents
From Command Mode to Expression Mode
RHS of Assignments
= and _ keywords
Expression Substitution
Arguments to Inline Function Calls
Parameter Lists
Oil if, while, and for
From Expression Mode to Command Mode
Array Literals
Command Substitution
Block Literals
Examples
How Are Glob Patterns Written in Each Mode?

Here is a list of places we switch modes.

From Command Mode to Expression Mode

RHS of Assignments

Everything after = is parsed in expression mode:

var x = 42 + f(x)    # RHS of var/setvar
setvar x += g(y)

setvar x = obj.method()   

x = 'myconst'

= and _ keywords

Likewise, everything after = or _ is in expression mode:

= 42 + f(x)

Throw away the value:

_ L.append(x)

Expression Substitution

echo $[42 + a[i]]

Arguments to Inline Function Calls

echo $strfunc(1, 2, a[i])
echo @arrayfunc('three', 'four', f(x))

Parameter Lists

proc p(x, y) {  # what's between () is in expression mode
  echo $x $y    # back to command mode
}

Oil if, while, and for

Expressions appear inside ():

if (x > 0) { 
  echo positive
}

while (x > 0) {
  setvar x -= 1
}

for (k, v in mydict) { 
  echo $x $y
}

From Expression Mode to Command Mode

Array Literals

var myarray = %( /tmp/foo ${var} $(echo hi) @myarray )

Command Substitution

Everything in between sigil pairs is in command mode:

var x = $(hostname | tr a-z A-Z) 

var y = @(seq 3)   # Split command sub

Block Literals

var b = &(echo $PWD)

Examples

How Are Glob Patterns Written in Each Mode?

No:

echo '*.py'              # a literal string, not a glob

echo @glob(*.py)         # syntax error, * is an operator in 
                         # expression mode

var x = myfunc(*.py)     # ditto, syntax error

Yes:

echo *.py                # expanded as a glob

echo @glob('*.py')       # A literal string passed to the builtin
                         # glob function

var x = f('*.py')        # Just a string

var x = f(glob('*.py'))  # Now it's expanded

Another way to say this is that Oil works like Python:

from glob import glob
glob('*.py')             # this is a glob
os.listdir('*.py')       # no glob because it's not how listdir() works

Also note that Oil has a builtin operator that uses glob aka fnmatch() syntax:

if (x ~~ '*.py') {  # not yet implemented
  echo 'Python'
}

Generated on Wed Dec 29 23:52:09 EST 2021