Prefix/Postfix Expressions

[mattcce]

CSEC: Prefix/Postfix Expressions

This PR aims to implement prefix/postfix expressions for a restricted subset of possible lvalues for the CSEC machine.

The CSEC machine presently does not have any implementation available for any prefix/postfix expression kind.

Lvalues

Common lvalues (read: may not be an exhaustive list) in Java include:

• variable names,
• array access expressions, and
• field access expressions.

This PR only targets prefix/postfix expressions for variable names. The other lvalue types are not in scope for this PR.

Prefix Expression

A prefix expression:

1. performs the operation and assignment, then
2. evaluates to the final value referenced by the lvalue.

• Preconditions: None.
• Postconditions:
  1. the lvalue has its value set to the value after applying the relevant binary operation to the original value.
  2. the top of the stash contains the final value referenced by the lvalue.

To implement this, we give prefix expressions the control expansion:

• C, PrefixExpression -> C, <name>, <assignment-statement>

where:

• <name> is the variable name that occurs in the prefix expression, and
• <assignment-statement> is the equivalent updating assignment statement required by the prefix expression.

Postfix Expression

A postfix expression:

1. performs the operation and assignment, then
2. evaluates to the original value referenced by the lvalue.

• Preconditions: None.
• Postconditions:
  1. the lvalue has its value set to the value after applying the relevant binary operation to the original value.
  2. the top of the stash contains the original value referenced by the lvalue.

To implement this, we give postfix expressions the control expansion:

• C, PostfixExpression -> C,<assignment-statement>, <name>

where:

• <name> is the variable name that occurs in the postfix expression, and
• <assignment-statement> is the equivalent updating assignment statement required by the postfix expression.

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Original implementation by @Isha-Sovasaria, changes have since gone stale.

Will aim to take over this feature from here on out.