7Interpreter

7.1How Interpreter Works

The Interpreter looks up and modifies content of Environment in accordance with Expressions that has been generated by parsing source codes.

The execution of the Interpreter consists of two stages evaluation and assignment. In an evaluation stage, it looks up variables in Environment and do evaluation depending on the current expression. In an assingment stage, the Interpreter will add new variables or modify existing variables in Environment.

In the Interpreter, Evaluation stage always occurs on each Expression while Assignment stage only does when Assign expression is executed.

      +---------------+
      |  Expressions  |
      +---------------+
              | control
              V
+----------------------------+     look-up     +---------------+
|              evaluation <--|-----------------|               |
|  Interpreter      |        |   assignment    |  Environment  |
|                   `--------|---------------->|               |
+----------------------------+                 +---------------+

7.2Evaluation Stage

7.2.1Overview

This section explains how each Expression acts in the Interpreter's evaulation stage.

7.2.2Evaluation of Value

Evaluation result of a Value expression will be the value that it owns in itself.

Consider the following expressions:

• 3.141

  Returns an instance of number type.

• 'hello'

  Returns an instance of string type

• b'\x00\x01\x02\x03'

  Returns an instance of binary type.

7.2.3Evaluation of Identifier

An Identifier expression will look up a variable whose name matches the expression's symbol in an Environment and return the result value. If no variable is found, it occurs an error.

Consider the following expression:

• foo

  Looks up a symbol foo in the current Environment and returns the associated value if found. If the symbol does not exist, occurs an error.

7.2.4Evaluation of Suffixed

A Suffixed expression will look up an entry in Suffix Manager that matches its suffix symbol and execute the entry with its body string.

Consider the following expressions:

• 123.45foo
  1. Looks up a handler associated with a symbol foo in the Suffix Manager.
  2. If found, it evaluates the handler by passing it a string '123.45' and returns the result. If no handler is found, occurs an error.
• 'hello world'bar
  1. Looks up a handler associated with a symbol bar in the Suffix Manager.
  2. If found, evaluates the handler by passing it a string 'hello world' and returns the result. If no handler is found, occurs an error.

7.2.5Evaluation of UnaryOp

A UnaryOp expression evaluates the child expression it owns, and then evaluate the value with its associated unary operator.

Consider the following expressions:

• -123.45
  1. Evaluates the child expression and gets a value 123.45 of number type.
  2. Looks up a unary operator function of - that can calculate number type.
  3. Evaluates the function by passing it a number 123.45 and returns the result.

7.2.6Evaluation of Quote

A Quote expression

`X

7.2.7Evaluation of BinaryOp

A BinaryOp expression evaluates both of the two child expressions it owns, and then evaluate the value with its associated Binary Operator.

X + Y

Binary Operator && and || are exceptional.

With operator &&, it first evaluates the child expression on the left. If the value is determined as false, that value is the result. Otherwise, it then evaluates the child expression on the right and returns the result.

With operator ||, it first evaluates the child expression on the left. If the value is determined as true, that value is the result. Otherwise, it then evaluates the child expression on the right and returns the result.

7.2.8Evaluation of Assign

Execution of an Assign expression triggers Assignment Stage. See the next section.

X = Y

7.2.9Evaluation of Member

A Member expression

X.Y

Class, Module and Object

7.2.10Evaluation of Lister

A Lister expression

[A, B, C]

7.2.11Evaluation of Iterer

An Iterer expression

(A, B, C)

7.2.12Evaluation of Block

A Block expression

{A, B, C}

7.2.13Evaluation of Root

A Root expression

7.2.14Evaluation of Indexer

An Indexer expression

X[A, B, C]

x[2]

x[1, 2, 3]

x['foo']

How an Indexer expression behaves in Interpreter's evaluation and assignment stage depends on what instance the car element returns.

If car's instance is of list type:

• Evaluation: the expression seeks the list's content at specified positions by indices.
• Assignment: modifies or adds the list's content at specified positions by indices.

In these cases, indices values are expected to be of number type.

If car's instance is of dict type:

• Evaluation: the expression seeks the dictionary's content using indices as the keys.
• Assignment: modifies or adds the dictionary's values associated with specified keys by indices.

In these cases, indices values are expected to be of number, string or symbol type.

7.2.15Evaluation of Caller

A Caller expression evaluates expressions listed as its arguments.

X(A, B, C)

f(a, b, c, d)

f(a, b):foo:bar

f {}

If the argument is declared as Quoted, it doesn't evaluates its argument.

How a Caller expression behaves in Interpreter's evaluation stage depends on what instance the car element returns.

If car's instance is of function type the expression calls the function with specified arguments.

If the Caller expression is specified as a target in Interpreter's assignment stage, it always creates function instance and assigns it in a specific Environment.

7.3Assignment Stage

7.3.1Overview

In an operation X = Y, the target expression X may be one of Identifer, Lister, Member, Indexer and Caller.

If the target expression is Identifier, Lister or Member, the source expression is evaluated at first before the result is assigned to the target.

If the target expression is Caller, the source expression itself is assigned to the target without any evaluation.

7.3.2Assignment for Identifier

An assignment for an Identifier expression

X = Y

If a type name is specified as the Identifier's attribute, the source value will be casted to the type before assignment.

a:number = '3'

This works in the same way as a data type casting in an argument list of function call. See Chapter.7. Function for more detail.

7.3.3Assignment for Lister

When the assignment destionation is a Lister expression, assignment operation is applied to each expression described as its element. Elements in the Lister must be Identifier expressions.

[A, B, C] = X

If assignment source is a scalar, that value is assigned to each element.

[a, b, c] = 3          // a = 3, b = 3, c = 3

If assignment source is a list, each value in the list is assigned to each element.

[a, b, c] = [1, 2, 3]  // a = 1, b = 2, c = 3

It would be the same with an iterator.

[a, b, c] = (1, 2, 3)  // a = 1, b = 2, c = 3

If the assignment source has more elements than the destination requires, remaining elements are simply ignored. If the source has insufficient number of elements, it would occur an error.

[a, b, c] = [1, 2, 3, 4, 5]  // a = 1, b = 2, c = 3
[a, b, c] = [1, 2]           // error!

7.3.4Assignment for Member

A Member expression

X.Y = Z

Class, Module and Object

obj.var1 = 3
obj.f(x) = { }

7.3.5Assignment for Indexer

An Indexer expression

X[A] = Y

X[A, B, C] = Y

x[n] = y

x[n] = 3

x[0, 2, 5] = 3

x[0, 2, 5] = [1, 2, 3]

7.3.6Assignment for Caller

A Caller expression

X(A, B, C) = Y

Assignments for other expressions than what are described above are invalid and occurs an error.

7.3.7Operator before Assignment

An Assignment operator can be combined with one of several other operators.