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Variable Declaration, Mutation, and Scope

This doc addresses these questions:

• How do variables behave in Oil?
• What are some practical guidelines for using them?

Oil Design Goals

The Oil language is a graceful upgrade to shell, and the behavior of variables follows from that philosophy.

• Oil implements shell-compatible behavior.
• It enhances shell with new features like expressions over typed data, which will be familiar to Python and JavaScript programmers.
• It's a stricter language.
  • Procs (shell functions) are self-contained and modular. In other words, they're understandable by reading their signature.
  • We removed dynamic scope. This mechanism isn't familiar to most programmers, and may cause accidental mutation (bugs).
  • Oil has variable declarations like JavaScript, which can prevent trivial bugs.
• Even though Oil is stricter, it should still be convenient to use interactively.

Keywords Are More Consistent and Powerful Than Builtins

Oil has 5 keywords affect shell variables. Unlike shell builtins, they're statically-parsed, and take dynamically-typed expressions on the right.

Declare With var and const

This is similar to JavaScript.

proc p {
  var name = 'Bob'
  const age = (20 + 1) * 2
  echo "$name is $age years old"  # Bob is 42 years old
}

Mutate With setvar and setglobal

proc p {
  var name = 'Bob'       # declare
  setvar name = 'Alice'  # mutate

  setglobal g = 42       # create or mutate a global variable
}

"Return" By Mutating Out Params With setref (advanced)

"Out Params" are a more controlled version of shell's dynamic scope. They reuse the nameref mechanism.

proc p(s, :myout) {           # declare out param with :
  setref myout = "prefix-$s"  # mutate it to "return" value to caller
}

• Style guideline: In some situations, it's better to "return" a value on stdout, and use $(myproc) to retrieve it.
• Implementation detail: The out parameter name is "hidden" with a __ prefix. This avoids a problem with nameref cycle detection.

Comparison to Shell

Shell and bash have grown many mechanisms for "declaring" and mutating variables:

• "bare" assignments like x=foo
• builtins like declare, local, and readonly
• The -n "nameref" flag

Examples:

readonly name=World        # no spaces allowed around =
declare foo="Hello $name"
foo=$((42 + a[2]))
declare -n ref=foo         # $foo can be written through $ref

These constructs are all discouraged in Oil code.

Keywords Behave Differently at the Top Level (Like JavaScript)

The "top-level" of the interpreter is used in two situations:

1. When using Oil interactively.
2. As the global scope of a batch program.

Experienced Oil users should know that keywords like var behave differently in the top-level scope vs. proc scope. This is due to the tension between shell's interactive nature and Oil's strictness (and to the dynamic nature of the source builtin).

For reference, JavaScript's modern let keyword has similar behavior.

Usage Guidelines

Before going into detail on keyword behavior, here are some practical guidelines:

• Interactive sessions: Use setvar only. This keyword is like Python's assignment operator: it creates or mutates a variable.
  • Short scripts (~20 lines) can also use this style.
• Long programs: Refactor them into composable "functions", i.e. proc.
  • First wrap the whole program into proc main(@argv).
  • Declare all variables with var and const.
  • Inside procs, use setvar to mutate local variables, and setglobal to mutate globals.
  • The top level should only have const declarations. (You can use var, but it has special rules, explained below.)

That's all you need to remember. The following sections explain the rationale for these guidelines.

The Top-Level Scope Has Only Dynamic Checks

The lack of static checks affects the recommended usage for both interactive sessions and batch scripts.

Interactive Use: setvar only

As mentioned, you only need the setvar keyword in an interactive shell:

oil$ setvar x = 42   # create variable 'x'
oil$ setvar x = 43   # mutate it

Details on top-level behavior:

• var behaves like setvar: It creates or mutates a variable. In other words, a var definition can be redefined at the top-level.
• A const can also redefine a var.
• A var can't redefine a const because there's a dynamic check that disallows mutation (like shell's readonly).

Batch Use: const only

It's simpler to use only constants at the top level.

const USER = 'bob'
const HOST = 'example.com'

proc p {
  ssh $USER@$HOST ls -l
}

This is so you don't have to worry about a var being redefined by a statement like source mylib.sh. A const can't be redefined because it can't be mutated.

It may be useful to put mutable globals in a constant dictionary, as it will prevent them from being redefined:

const G = {
  mystate = 0
}

proc p {
  setglobal G->mystate = 1
}

proc Scope Has Static Checks

Procs are Oil's stricter notion of "shell functions", and they have additional static checks (parse errors):

• Every variable must be declared once and only once with var or const. A duplicate declaration is a parse error.
• Mutating a const is a parse error.
• setvar of an undeclared variable is a parse error.

Procs Don't Use "Dynamic Scope"

Procs are designed to be encapsulated and composable like processes. But the dynamic scope rule that Bourne shell functions use breaks encapsulation.

Dynamic scope means that a function can read and mutate the locals of its caller, its caller's caller, and so forth. Example:

g() {
  echo "f_var is $f_var"  # g can see f's local variables
}

f() {
  local f_var=42
  g
}

f

Oil code should use proc instead. Inside a proc call, the dynamic_scope option is implicitly disabled (equivalent to shopt --unset dynamic_scope).

Reading Variables

This means that adding the proc keyword to the definition of g changes its behavior:

proc g() {
  echo "f_var is $f_var"  # Undefined!
}

This affects all kinds of variable references:

proc p {
  echo $foo         # look up foo in command mode
  var y = foo + 42  # look up foo in expression mode
}

As in Python and JavaScript, a local foo can shadow a global foo. Using CAPS for globals is a common style that avoids confusion. Remember that globals should usually be constants in Oil.

Shell Language Constructs That Write Variables

In shell, these language constructs assign to variables using dynamic scope. In Oil, they only mutate the local scope:

• x=val
  • And variants x+=val, a[i]=val, a[i]+=val
• export x=val and readonly x=val
• ${x=default}
• mycmd {x}>out (stores a file descriptor in $x)
• (( x = 42 + y ))

Builtins That Write Variables

These builtins are also "isolated" inside procs, using local scope:

• read ($REPLY)
• readarray aka mapfile
• getopts ($OPTIND, $OPTARG, etc.)
• printf -v
• unset

Oil Builtins:

• compadjust
• run --assign-status

Reminder: Proc Scope is Flat

All local variables in shell functions and procs live in the same scope. This includes variables declared in conditional blocks (if and case) and loops (for and while).

proc p {  
  for i in 1 2 3 {
    echo $i
  }
  echo $i  # i is still 3
}

It also includes Oil's first-class blocks:

var x = 42
cd /tmp {
  var x = 0  # ERROR: x is already declared
}

More Details

Examples of Place Mutation

The expression to the left of = is called a place. These are basically Python or JavaScript expressions, except that you add the setvar or setglobal keyword.

setvar x[1] = 2                 # array element
setvar d['key'] = 3             # dict element
setvar d->key = 3               # syntactic sugar for the above
setvar x, y = y, x              # swap

Bare Assignment

shopt --set parse_equals is currently off in both OSH and Oil. It allows constants to be declared without the const keyword:

const x = 'foo'

x = 'foo'  # Similar.  This is NOT a mutation as in C or Java.

However, it doesn't do a static check for 'const' already defined in proc. It will be used for future config-like use cases:

subdomain app.example.com {  # subdomain proc takes a string and block
  root = '/home/www/bin/'    # no var or const keyword necessary
}

subdomain docs.example.com {
  root = '/home/www/docs/'   # not a redefinition
                             # blocks may or may not introduce a new scope
}

When it's enabled, x=foo (no spaces) is disallowed to prevent confusion. Use the env command instead:

env PYTHONPATH=. ./foo.py  # good
PYTHONPATH=. ./foo.py`.    # disallowed because it would be confusing

Temp Bindings

Temp bindings precede a simple command:

PYTHONPATH=. mycmd

They create a new namespace on the stack where each cell has the export flag set (declare -x).

In Oil, the lack of dynamic scope means that they can't be read inside a proc. So they're only useful for setting environment variables, and can be replaced with:

env PYTHONPATH=. mycmd
env PYTHONPATH=. $0 myproc  # using the ARGV dispatch pattern

Appendix A: More on Shell vs. Oil

This section may help experienced shell users understand Oil.

Shell:

g=G                        # global variable
readonly c=C               # global constant

myfunc() {
  local x=X                # local variable
  readonly y=Y             # local constant

  x=mutated                # mutate local
  g=mutated                # mutate global
  newglobal=G              # create new global

  caller_var=mutated       # dynamic scope (Oil doesn't have this)
}

Oil:

var g = 'G'                # global variable (discouraged)
const c = 'C'              # global constant

proc myproc {
  var x = 'L'              # local variable
  const y = 'Y'            # local constant

  setvar x = 'mutated'     # mutate local
  setglobal g = 'mutated'  # mutate global
  setvar newglobal = 'G'   # create new global

                           # There's no dynamic scope, but you can use
                           # "out params" with setref.

}

Appendix B: Problems With Top-Level Scope In Other Languages

• Julia 1.5 (August 2020): The return of "soft scope" in the REPL.
  • In contrast to Julia, Oil behaves the same in batch mode vs. interactive mode, and doesn't print warnings. However, it behaves differently at the top level. For this reason, we recommend using only setvar in interactive shells, and only const in the global scope of programs.
• Racket: The Top Level is Hopeless
  • From A Principled Approach to REPL Interpreters (Onward 2020). Thanks to Michael Greenberg (of Smoosh) for this reference.
  • The behavior of var at the top level was partly inspired by this paper. It's consistent with bash's declare, and similar to JavaScript's let.

Related Documents

• Oil Keywords
• Interpreter State
  • The shell has a stack of namespaces.
  • Each namespace contains {variable name -> cell} bindings.
  • Cells have a tagged value (string, array, etc.) and 3 flags (readonly, export, nameref).
• Unpolished details: variable-scope.html and proc-block-func.html