CPS710 Assignment 5

CPS710	Assignment 5
	Semantics of HL - Part 2

This page contains additional HL semantics for this assignment. Note that they will not explicitly be enforced in this assignment, but in the next one as part of error management.

Declarations and Types
Scoping
Loops
Set Formers
General Function Call Evaluation Rules
Parameter Passing
Return Statements

Declarations and Types

HL is strongly typed. Each declared variable in a scope must evaluate to a value of the same type. As you already know from previous assignments, the type of a variable is implied by its name. In assignment 3, at the A level the parser nodified the three different types of identifier tokens into a single ASTIdentifier node with one child for the type. Make sure that the parser you are using also does that. The solution parser provided with Assignment 4 has this functionality.
In HL, only numbers, booleans, sets and functions can have names. Variables and functions can be declared to be of type NUM, BOOL, or SET.
The type of a function is the type of the value returned when the function is called.
A NUM variable has a default value of 0 and a SET has a default value of {}. BOOLs don't have default values.

Scoping

For the C level learning objective HL has a monolithic block structure (i.e. a single global scope) HL is dynamically scoped for learning objective B, and statically scoped for learning objective A. In all cases:
In each scope, variables must be declared before they are used.
In each scope, functions must be defined before they are used.

In static scoping, there are no scope blocks for loops, only functions (and the global scope, of course).

HL defines one additional special scope: in a set former expression, the set former variable is implicitly declared as a NUM variable whose scope is the condition of set former only, i.e. between the vertical bar and the closing curly bracket of the set former. Note that this scope does not include the set former's domain because the definition would end up being self-referential.

Loops

While: The semantics of while loops are the same as in other programming languages.
For-in:The semantics of loops with the structure:
for idnum in expression list do body end;
are:
1. idnum must have been defined as an IDNUM (i.e. it starts with a lower case letter) before is used in this loop.
2. the expressions in expression list will be evaluated one by one, and each time one of them is evaluated the following happens:
  - If the value is a NUM, idnum is assigned that value, and body is evaluated once.
  - If the value is a SET, idnum is assigned the values of the set one by one in order. The body is evaluated each time idnum takes a new value.
These semantics mean that the following two code snippets produce the same result:
Loop1:
for i in {1,2,3},4,{5} do println i; end;
Result:
1 2 3 4 5
Loop 2:
for i in 1,2,{3,4},5 do println i; end;
Result:
1 2 3 4 5
However, these two code snippets will not because in HL sets are ordered:
Loop1:
for i in 5,4,3,2,1 do println i; end;
Result:
5 4 3 2 1
Loop 2:
for i in {5,4,3,2,1} do println i; end;
Result:
1 2 3 4 5

Set Formers

Sets defined using the set former structure { idnum in expression | condition } are evaluated as follows:

idnum is the set former variable. It is declared as a NUM variable whose scope is the condition of the set former only, i.e. between the vertical bar and the closing curly bracket of the set former. Its lifetime ends when the set has been evaluated.
expression is evaluated first. It must evaluate to a set
for each value of this set:
- idnum is assigned the value,
- the condition, which may refer to idnum, is evaluated
- if the condition evaluates to true, the value is in the set being formed. If it evaluates to false, it is not in the set.

General Function Call Evaluation Rules

Function are evaluated as follows:

The parameters are evaluated (see next section).
The function body is evaluated with all references to the parameters replaced by their value.
This evaluation must be ended by a return statement.

They are used in the following context:

If a function call is used as a statement rather than an expression, its return value is ignored by the clause or body where it is used.

Parameter Passing

Parameters are always passed by value, and their value cannot be changed during the execution of the function call.
Parameters are evaluated from left to right.
Parameters defined as BOOL, NUM, or SET must evaluate to a Boolean, HLNumber, or HLSet respectively.

Return Statements

Return statements are only meaningful inside function definitions. Return statements which are not inside these definitions are syntax errors. This will be checked in assignment 6.

A return statement which is inside nested function definitions applies to the innermost function definition, as is standard in most programming languages. For example, in the program:

return x; // this is an invalid return
function a()
  function b()
		...
		return y; // this is a return from b
		end;
	...
	return z; // this is a return from a
	end;

The parameter of the return statement, when there is one, must evaluate to a value of the same type as the return type as the function. The value of that parameter is the value of the entire function call.

When the return statement doesn't have any parameters, the value of the entire function call is null.

This page is maintained by Sophie Quigley (cps710@cs.torontomu.ca)
Last modified Thursday, 28-Nov-2024 12:28:02 EST

CPS710

Assignment 5

Semantics of HL - Part 2

Contents

Declarations and Types

Scoping

Loops

Set Formers

General Function Call Evaluation Rules

Parameter Passing

Return Statements

Declarations and Types

Scoping

Loops

Set Formers

General Function Call Evaluation Rules

Parameter Passing

Return Statements