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Variables

A variable is a way of holding on to and referencing the result of an expression. A variable points to an object and permits that object to be saved and repeatedly used later.

There are two types of variables: local variables and thread variables. The type of the variable defines its scope and the rules about using it. Each variable is given a name and that name is used to access the variable's value. A object which a variable points to can be changed, or reassigned, as described in the chapter Operators.

  • Variable Names describes the characters permitted in variable names

  • Local Variables are specific to each method call

  • Thread Variables are specific to each thread

  • Type Constraints describes how variable values can be restricted to certain object types

  • Decompositional Assignment describes how container objects can be split into multiple variables in one step

Variable Names

Lasso variable names can begin with an underscore, or the characters A-Z and then can be followed by zero or more underscores, letters, numbers or period characters. Character case is irrelevant when accessing a variable.

Local Variables

Each method call runs with its own set of variables. These are called local variables or locals, and they are the most commonly used type of variable. Locals begin and end within the method in which they are defined, though the objects they point to may exist beyond that point.

A local must be defined before it can be used. When a variable is defined, it is generally done so along with an initial value to be assigned to that variable. If an initial value is omitted, the variable will have the default value of null. Multiple locals can be defined at one time, either with or without default values, using the following syntax examples.

  local(name = expression)  defines local "name"
  local(name, b = 1)  defines local "name" and "b"

The first of the above sets the variable name to the value of the expression. The second defines name without a value and defines b with the value of 1.

A local can be accessed using two different methods. In the first method, the local variable may or may not have previously been defined. If the local has not been defined, then it is defined and assigned a value of null. Regardless, the value of the variable is produced as the result. This is only the case when one variable name is used and when it is not accompanied by an initial value.

  local(name)  the value of "name", potentially creating "name"

Local variables can also be accessed using the # symbol before the name. This is generally the most common method for accessing local variables.

  #name  the value of "name"

When using this method, the local variable must have already been defined or it is considered an error. This error checking is done at the time the code is parsed, meaning that the local definition must "physically" precede the # access point within the source code.

The set of local variables for each method is determined as the code is compiled, and can not be modified at runtime, unlike thread variables, which can be given names dynamically.

Parameter Pseudo-locals

Lasso permits the parameter values given to a method to be accessed by position using the local variable symbol # followed by an integer value. The integer value corresponds to the position of the desired parameter value, beginning with 1. For example, in a method given two parameters, the first would be available using #1 and the second would be available using #2.

See the chapter Defining Methods for information on methods and method parameters.

Thread Variables

Thread variables, or vars, are variables that are shared and accessible outside of any particular method call, yet are restricted to the currently executing thread. Each thread maintains its own set of vars. Vars are useful for maintaining program states which go beyond the operation of any one method.

Vars are created similarly to locals.

  var(name = expression)  defines var "name"
  var(name, b = 1)  defines var "name" and "b"

A var created without an initial value will be given the default value of null.

Vars can be created using an expression value for a name, unlike locals which require a fixed literal name. This expression must result in a string or a tag object. That value is used as the variable's name.

  var(nameExpr = expression)  defines var with name of nameExpr

Syntax Note: Because a literal variable name can resemble a method call with no parameters, if the variable name is intended to be the result of a method call then that call should be given empty parentheses () to disambiguate.

  var(nameCall() = expression)  defines var with name of what nameCall() returns

A var can be accessed using two methods, similar to that of local variables. First, the var may simply be referenced using the var construct along with the var's name. The var may or may not have previously been defined. If the var has not been defined, then it is defined and assigned a value of null. The value of the variable is produced as the result. This is only the case when one variable name is used and when it is not accompanied by an initial value.

  var(name)  the value of "name", potentially creating "name"

Vars can also be accessed using the $ symbol before the name. When using this method, it is an error if the var has not been previously defined.

  $name  the value of "name"

Type Constraints

A type constraint can be applied to a local or thread variable in order to ensure that the variable value is always of a particular type. For example, a local variable could be constrained to always hold a string object. If an attempt was made to assign to that variable a non-string object, such as an integer, the assignment would fail.

Lasso is a dynamically typed language, and, by default, variables can hold any type of object. Type constraints permit a developer to restrict variables to hold only particular object types in order to ensure that the code operating on those variables is working with valid inputs.

Type constraints are applied when a local or thread variable is first defined. This is done by supplying a tag literal, consisting of two colons :: and then the name of the type to which the variable will be constrained, immediately following the variable name. The following example applies type constraints to a local and a var.

  local(lname::integer = 0)
  var(vname::string = '')

#lname is constrained to hold only integers, and $vname is constrained to hold only strings. The next example shows valid and invalid usage of the two variables.

  #lname = 400  valid: 400 is an integer
  #lname = 'hello'  FAILURE: #lname can only hold integers

  $vname = 940  FAILURE: $vname can only hold strings

  local(lname = 'hello')  FAILURE: #lname can only hold integers

When applying a type constraint to a variable, it is required that a default value be provided.

  local(lname::integer, x, y, z)  FAILURE: #lname requires default value

Decompositional Assignment

Lasso will "decompose" the right-hand value (RHS, rvalue) of an assignment when the left-hand side (LHS) is a 'local' declaration containing just a list of variable names. This supports wildcards (the _ character) as well as nested name lists. Any type which supports trait_forEach can be used like this on the RHS.

Examples:

local(one, two, three, four) = (:1, 2, 3, 4, 5, 6)
#one 1
#two 2
#three 3
#four 4

local(_, two, _, four) = (:1, 2, 3, 4, 5, 6)
#two 2
#four 4

local(_, two, _, four) = 1 to 100 by 3
#two 4
#four 10

local(one, _, three, (_, four)) = array('a', 'b', 'c', array('d', 'e'))
#one #three #four ace

local(wanted, _, w2) = 'ABCDEFGH'
#wanted A
#w2 C

Note that the local must have more than one element in it and none of the elements can be given values.

local(x) = #foo unchanged, works as expected
local(x, _) = #foo fine. grabs first #foo
local(x = 1, _) = #foo FAILURE: x can not have value

Also note that assign-produce can not be used here := and that quoted variable names are not permitted.

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