Agenda. 1. Quiz 2. (Short) Quiz Discussion 3. Finish off Type-Bound Procedures 4. Semcheck2 Questions 5. Coding Tips 6. Phase II Suggestions

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1 Agenda 1. Quiz 2. (Short) Quiz Discussion 3. Finish off Type-Bound Procedures 4. Semcheck2 Questions 5. Coding Tips 6. Phase II Suggestions Timeline You should be finished with phase I and have begun implementing phase II. The SemCheck2 assignment is due in less than three weeks. Phase II and phase III are harder than phase I. We will discuss various implementation ideas and software engineering ideas for the next 3-4 lectures and begin discussing codegen & optimization in week 5. 1

2 What about these rules: TBP Calls Factor -> Designator (OptExprList) No change Designator -> Designator T_DOT T_ID Already checking this with records. Now ID may be in the procedure list, rather than the fields list. What do you pass up? Another TBP Example TYPE Student = RECORD id : INTEGER; name : ARRAY 10 OF CHAR; classes: ARRAY 10 OF CLASS;... END Student Procedure (rec: Student) GetID() return rec.id END GetID Procedure (another Rec: Student) SetId (newid : INTEGER) anotherrec.id := newid END SetID 2

3 Second TBP Example in Action VAR Drone : Student; VAR Borg; Student; Drone.SetId(44); Borg.SetId(66); WriteInt (Drone.GetID()); (* Writes 44 *) WriteInt (Borg.GetId()); (* Writes 66 *) Syntax and Semantics How do you get syntax-directed definitions to be your friend? How does it cause problems for you? There are a few places in the grammar that cause these problems look out for them! Does this affect ASTs? 3

4 The Challenge of a Big Spec How do you deal with a large, changing spec, with lots of code, a partner that you work with, and deadlines to meet? Incremental Development Coding to the Spec Incremental Development One type check at a time One syntax rule at a time One type at a time 4

5 Coding to the Spec ("Top to Bottom") Read the spec Identify which rules the check applies to Decide where the functions need to go and what there operands & results are. Implement pseudo-code that looks like the spec. Maybe just a high-level method. Replace it with real code. Implement the details. Coding to the Spec Example If you have a spec that looks like this: MyList [nindex]...where MyList needs to be a VAR of type Array and nindex needs to be an integer (const, var) How do you do this? Find this rule: Designator2 ::= Designator2 T_LBRACKET ExprList T_RBRACKET 5

6 Coding to the Spec Example Designator2 ::= Designator2 T_LBRACKET ExprList T_RBRACKET {: $$ = CheckCorrectArray($1, $3); :} Dealing with Records Let's say you have this code: TYPE Student = RECORD id : INTEGER grades : ARRAY 4 OF REAL END VAR CompilerClass : ARRAY 200 OF POINTER TO Student 6

7 Array Example How do we reference something in CompilerClass? CompilerClass[0].id = 10...gets parsed like: AssignStmt -> Designator Assign Expr Expr -> Factor -> INT_LITERAL Designator -> Designator[Expr] Designator -> Designator.ID Walk through the Rules RHS Expr -> Factor /* (from Phase I) */ $$ = $1; Factor -> INT_LITERAL $$ = new ConstSym($1); $$ ->type = integer; LHS Designator -> ID $$ = symboltable->lookup ($1); /* normally */ 7

8 Steps for Arrays 1. Verify that Designator (Des) is in the SymTable (last quarter) and is a Var. 2. Verify that Des's type is an array 3. Verify that Expr here can be used. Can come back from Expr as Var, Const, or Error. 4. Type of Expr (the array index) must be int. 5. Must use integer indexes on arrays in Oberon. 6. Create a SymObject to send back. What kind of SymObject? Steps for Records Des -> Des. ID 1. Verify that $1 is a variable. 2. Verify that $1's type is a pointer. 3. Verify that $1's type points to a record. 4. Verify that ID is in the fields list of $1's record?? 5. Create a SymObject to send back. 8

9 Another Example CompilerClass.grades[3] := 3.0;...gets parsed like: AssignStmt -> Des Assign Expr Expr -> Factor -> REAL_LITERAL Des -> Des. ID Des -> Des [ Expr ] How is it parsed? Expr -> Factor -> REAL Yes, we know from phase I Des -> Des. ID Is $1 a var? Is $1's type record? NO. Give error to user. Now what? 9

10 Example 3 VAR MyGrade : ARRAY 5 OF REAL; CompilerClass[0].grades := MyGrade In AssignStmt -> Des Assign Expr 1. Is Des a VAR? 2. Is $1->type and Assignable? Arrays of Arrays Example TYPE y = ARRAY 10 OF INTEGER; TYPE x = ARRAY 10 of y; VAR z : x; z[1][2] := 10; 10