Project Design. Error Format

Transcription

1 Project Design Designing for the future is important. In the real world, "scope creep" is inevitable. In our world of CSE 131B, specifications are clarified and altered slightly as time goes by and questions come up. Error Format File test.obe, Line 10: syntax error. If it says anything anywhere else, please disregard. Use this format. There is a new file up which specifies the way you print error messages differently for different types: s.txt 1

2 Error Type Use Error Type for error suppression, rather than stopping compilation. As with ErrorSyms. Example: VAR x, y : INTEGER; VAR b : BOOLEAN; BEGIN x := (x + b) + y; END. Reasonable solution. The error is in :=. What happens with the errors? x := (x + b) + y; (x + b) Causes an error to be reported and an ErrorType returned. ONLY for (x+b). What about the rest of the expression? 2

3 What happens with multiple errors? x := (x + b) + y + b; (x+b) is an ErrorType (and generates an error). But the ErrorType should be able to "play nicely" with the rest of the expression and masquerade as or change into an integer or boolean. The first part of the expression becomes an integer. When you hit the second b, you have another error. Rewriting Expr Only to help with semcheck and generating code. If you're already using an AST, you don't need to worry about rewriting the grammar. Actually, with an AST, you can handle this: TYPE me = RECORD END;... you : POINTER TO me; 3

4 Type Rules Numeric Type = INTEGER and REAL Same Type If two variables a and b with types Ta and Tb are such that Ta and Tb are both denoted by the same type identifier. Type Inclusion Numeric types include the values of smaller numeric types. In our case, INTEGER is a subset of REAL. Assignment Compatible An expression like this: T2 := T1...is assignment compatible (i.e. T1 is assignable to T2) iff: T1 and T2 have equivalent types T1 and T2 are numeric types and T2 includes T1. real >= int r := i (* ok *) i := r (* not ok *) 4

5 When to Use What? In a procedure call, you have to compare the actual parameters against the formal parameters. For VAR parameters, use assignability For non-var parameters, use equivalence. Expression Compatible operator first operand second operand Result Type + - * numeric numeric smallest numeric type, including both operands / numeric numeric REAL DIV MOD INTEGER INTEGER INTEGER OR & ~ BOOLEAN BOOLEAN BOOLEAN = # < > numeric numeric BOOLEAN = # BOOLEAN BOOLEAN BOOLEAN 5

6 Name Equivalence Two types are name equivalent if they have the same "name" type R = RECORD; var A : R; var B, C : R; A, B, and C are all Name Equivalent (NE). Not Name Equivalence var X : RECORD END var Y, Z : RECORD END Y & Z are NE. X & Z are not NE. X & Y are not NE. X & Y look the same, but are not NE. What is anonymous type? 6

7 Structural Equivalence Is this "whole" type equal to that "whole" type? VAR a1 : ARRAY 10 OF INTEGER; VAR a2 : ARRAY 10 OF INTEGER; VAR a3 : ARRAY 10 OF REAL; VAR a4 : ARRAY 20 of INTEGER; None are NE. All point to separate anonymous type structures. A1 & A2 are SE. No other pair is SE. Another SE Example TYPE r1 = RECORD a, b : INTEGER END TYPE r2 = RECORD c, d : INTEGER END Are r1 & r2 SE? 7

8 Another SE Example TYPE r3 = RECORD a : REAL b : INTEGER END TYPE r4 = RECORD b : INTEGER a : REAL END Are r3 & r4 SE? Oberon Equivalence basic types (int/real/bool) : NE arrays: NE (we're not doing dynamic arrays, which are different) VAR a1 : ARRAY 10 OF INTEGER VAR a2: ARRAY 10 OF INTEGER NE? SE? OE? records: NE pointers: NE 8

9 alias types: SE Oberon Equivalence TYPE A = RECORD... TYPE B = A TYPE C = B TYPE D =C All Oberon Equivalent. a + b Expr -> Expr1 OP Expr2 Expr1 ->->->Factor->Designator Designator -> T_ID { $$ = symbol_table->access($1); } Expr1 & Expr2 must hold values Expr1 & Expr2 must be compatible Must pass up a value 9

10 Temps What do you do when you have: Expr -> Expr OP Expr2 { //Chck errors $$ =new VarSym("UniqueName"); $$->type = (applicable type from $1 & $3) Does this go in the symbol table? Temps What do you do when you have x+6: Expr -> -> -> Factor Factor -> T_INT_LITERAL { $$ = new ConstSym("UniqueName"); $$->type = int type (lookup or global ptr) Does this go in the symbol table? 10

11 Type-Bound Procedures Type-bound procedures are Oberon's way of creating classes by adding procedures to existing records. TYPE Student = RECORD id : INTEGER; grades : ARRAY 4 OF REAL; END TYPE StudentPtr = POINTER TO STUDENT; Type-Bound Procedures PROC (S: StudentPtr) AddScore (index : INTEGER, score: REAL)... END AddScore S represents the instance of the record used to call this procedure. How does this work in C++/Java/Smalltalk? 11

12 Type-Bound Procedure Usage Example VAR Sue : StudentPtr; VAR Joe : StudentPtr; Sue.AddScore(0,3.2); Joe.AddScore(1,2.3); Type-Bound Procedure Member Variables Within a type-bound procedure, the parent record's fields must be referenced through the receiver. PROC (S : StudentPtr) AddScore (index : INTEGER, score: REAL) grades[index] := score; (*grades undecl. *) S.grades[index] := score; (* correct *) S.AddScore (index+1, score * 2); (*Recurs. call *) END AddScore 12

13 TBP Reminders Type-Bound Procedure does not go into current scope (cannot be called without a var reference) Add ProcSym to the record's proc list. Insert receiver as a local var to the proc's scope. Optional receiver acts like Java/C++ this. In Oberon, the receiver can have any name. TBP Definition ProcDecl -> PROC OptReceiver ID (OptParamList) OptReturnType... 13

14 TBP Checks $5 (second ID) is a type $5 is a pointer. pass up what? Receiver ($3) is an ID, not a VarSym. $$ = new VarSym($3); $$->type = $5; TBP Calls What about these rules: 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? 14