Download Exceptions in Programming: Efficient Implementation and Use - Prof. Westley Weimer and more Study notes Programming Languages in PDF only on Docsity!
ACM Trivia Bowl
Today
• 7pm OLS 011
• Snacks and drinks provided
• All are welcome! I will be there.
– email eas2h with team or just show up
• If you are in one of the top three teams, I
will give you one point of extra credit (as
if you had found a bug in Cool).
“Wizard is about to die.”
- (^) PA5 is due Wednesday April 23 rd - that's about
three weeks from now.
- I have only one auto-tester submission as lunch
time today.
- I am predicting that you haven't started PA5 yet.
- You will have second midterms in this class
(and others!) between then and now.
- If you can't interpret hello-world.cl by the end
of this weekend, I forsee regret, remorse and
lack of sleep in your future.
One-Slide Summary
- Real-world programs must have error-
handling code. Errors can be handled where
they are detected or the error can be
propagated to a caller.
- Passing special error return codes is itself
error-prone.
- Exceptions are a formal and automated way
of reporting and handling errors. Exceptions
can be implemented efficiently and described
formally.
Language System Structure
- We looked at each stage in
turn
affects many stages
- We will add exceptions Source Lexer Parser Code Generator Runtime System Run it! Type checker
Exceptional Motivation
- “Classroom” programs are written with optimistic assumptions
- Real-world programs must consider “exceptional” situations: - Resource exhaustion (disk full, out of memory, network packet collision, …) - Invalid input - Errors in the program (null pointer dereference)
- It is usual for code to contain 1-5% error handling code (figures for modern Java open source code) - (^) With 3-46% of the program text transitively reachable
Why do we care?
- Are there any implications if software makes
mistakes?
Error Return Codes
- The callee can signal the error by returning a
special return value or error code :
- Must not be one of the valid inputs
- Must be agreed upon beforehand (i.e., in API)
- The caller promises to check the error return
and either:
- Correct the error, or
- Pass it on to its own caller
Error Return Codes
- It is sometimes hard to select return codes
- What is a good error code for:
- divide(num: Double, denom: Double) : Double { … }
- How many of you always check errors for:
- malloc(int)?
- open(char *)?
- close(int)?
- time(struct time_t *)?
- Easy to forget to check error return codes
Example: Automated Grade
Assignment
float getGrade(int sid) { float res; int err = dbget(gradesdb, sid, &res); if(err < 0) { return -1.0;} return res; } int extraCredit(int sid) { int err; float g = getGrade(sid); if(g < 0.0) { return 1; } err = setGrade(sid, 0.33 + g)); return (err < 0); }
Example: Automated Grade
Assignment
float getGrade(int sid) { float res; int err = dbget(gradesdb, sid, &res); if(err < 0) { return -1.0;} return res; } int extraCredit(int sid) { int err; float g = getGrade(sid); if(g < 0.0) { return 1; } err = setGrade(sid, 0.33 + g)); return (err < 0); } A lot of extra code
Example: Automated Grade
Assignment
float getGrade(int sid) { float res; int err = dbget(gradesdb, sid, &res); if(err < 0) { return -1.0;} return res; } int extraCredit(int sid) { int err; float g = getGrade(sid); if(g < 0.0) { return 1; } err = setGrade(sid, 0.33 + g)); return (err < 0); } Some functions change their type Error codes are sometimes arbitrary A lot of extra code
Exceptions
- Exceptions are a language mechanism
designed to allow:
- Deferral of error handling to a caller
- Without (explicit) error codes
- And without (explicit) error return code checking
Adding Exceptions to Cool
(Informal) semantics of try e catch x : T ) e
1
- e is evaluated first
- If e’s evaluation terminates normally with v then v is the result of the entire expression Else (e’s evaluation terminates exceptionally) If the exception parameter is of type · T then
- Evaluate^ e 1 with^ x^ bound to the exception parameter
- The (normal or exceptional) result of ev
- aluating^ e 1 becomes the result of the entire expression Else
- The entire expression terminates exceptionally
Example:
Automated Grade Assignment
float getGrade(int sid) { return dbget(gradesdb, sid); } void setGrade(int sid, float grade) { if(grade < 0.0 || grade > 4.0) { throw (new NaG); } dbset(gradesdb, sid, grade); } void extraCredit(int sid) { setGrade(sid, 0.33 + getGrade(sid)) } void grade_inflator() { while(gpa < 3.0) { try extraCredit(random()) catch x : Object ) print “Aie!?\n”; } }
