Variables & Scope

A variable is a name bound to a value with =. Assignment is a statement and it runs for its effect on the scopes the name is bound in not for a value like when evaluating an expression.

Quick reference

x = 1              # bind a name
x?                 # read leniently, <undefined> if unbound
list[0] = 9        # assign into a list element
map['key'] = 9     # insert or replace a map key
p.x = 42           # assign a property
this.x = 42        # assign a property inside a method

Binding a variable

The left hand side is a name, the right-hand side is any expression:

x = 1
greeting = 'hello'
total = x + 10

• There is no compound assignment write count = count + 1 not count += 1.

Because assignment is a statement print(x = 1) is an error not a print of 1. The target must be a name, a property, or an index. f() = 1 and x? = 1 are errors.

Index assignment

Index assignment targets a list, a map, or a JVM array of objects.

list = [1, 2, 3]
list[0] = 9

map = {'a' -> 1}
map['b'] = 2

On a map the index is the key, and assigning inserts it if it is not already there.

Targets chain grid[0][1] = 42 and o.kid.v = 7 are both legal.

• A call anywhere in the chain, as in s.split(',')[0] = 'z', is an error.
• A string can be read by index but not assigned into'foo'[0] is f, while s[0] = 'b' is a runtime error.
• A set is neither {1, 2, 3}[0] is null rather than the first member, and assigning into a set is a runtime error.

Property assignment

Assign to a property with dot access on the left hand side. Target must be an instance of a Notch class and the class must declare the field.

class Point
  field x = 0
end

p = new Point()
p.x = 42

Inside a method this is the root:

class Counter
  field n = 0
  function bump()
    this.n = this.n + 1
  end
end

• Assigning a property on a JVM object is a runtime error even though reading one works. The same Point shape from a Java class rejects the write:

import java.awt.Point as Pt
p = new Pt(5, 6)
print(p.x)     # 5.0
p.x = 9        # ERROR: cannot assign a property on a non-instance value

Scope

A name is visible from its binding onward within the enclosing scope.

n = 1
function bump()
  n = n + 1
end
bump()
print(n)
# prints 2

A scope is opened by a loop body, a function or closure body, a class method body, and a catch clause. An if or else body and a try body do not open one, so a name bound inside an if is still bound after its end.

Function declarations follow the same rule a function declared inside any of those scopes is not callable after that body ends and reports as undefined function rather than unknown variable.

• ? cannot rescue it.

for n in [1]
  function hidden()
    print('hi')
  end
end
hidden()    # undefined function 'hidden'
hidden?()   # same error '?' does not rescue a call

Contrast an if body, which opens no scope, so a function declared inside one is callable after its end.

A loop body is a single scope covering all iterations and any name first bound inside a loop is unbound once the loop ends.

total = 0
for n in [1, 2, 3]
  total = total + n
  x = "unbound"
end
print(x?) # prints undefined
print(total) # prints 0

A parameter hides an outer name while the body only reads it but assigning to it writes through to any binding in an enclosing scope:

n = 1
function f(n)
  n = n + 1
end
f(99)
print(n) # prints 100