Class 20: Type checking, continued

Held Wednesday, October 14, 1998

Notes

• Nathaniel Borenstein will be visiting campus the first two weeks of the second half of this semester.
  • As part of his visit, he is giving the convocation speech on October 29th. Typically, students are invited to a post-convocation lunch. If you are interested in attending that lunch, please let Mr. Walker know.
  • A very rough draft of his schedule is available online.
• There are a few small typos on the exam which most of you figured out.
  • In the last problem of the Java version, two page numbers are reversed.
  • In the C version, when it refers to "red book", read "green book".
  • In the last problem of the C version, where it says "put", read "S_enter".

Type checking, continued

Issues

• Recall that there are three things we want to do while type-checking:
  • Verify that all types are used correctly (e.g., that only integers are added).
  • Determine the types of expressions. This is necessary to verify that types are used correctly, since most expressions have subexpressions. For example, when we assign to a variable, we need to determine the types of both the variable and the assigned expression.
  • Verify that all identifiers are declared before use.

Errors and error recovery

• As is the case for other stages of the compiler, we'd like our type checker to be able to continue processing even after an error is encountered.
• For most expressions, it is possible to pick a ``default'' type, even when there is a conflict.
  • If the explicit type and expression type for a function or variable differ, report an error and use the explicit type.
  • If a variable is undeclared, either naively assume that it's an integer or determine its type from context (the latter is harder to do, as you may need to pass an ``expected type'' into the type checking mechanism).
  • If an arithmetic operator is applied to two subexpressions, assume the type of the arithmetic expression is integer.

Examples

• For some of the following, we'll summarize the expected check that is done.
• Note that we can do some error recovery along the way
• Nil
• Array constants
• Record constants
• Function calls
• Operator application
• Sequences
• Assignments
• Conditionals
• Loops
• Breaks
• For loops
• Let expressions
• We focused on array and record-related issues, including recovering variables from the previous class.

History

• Created Wednesday, October 14, 1998. Some portion of the outline was taken from the previous day's outline.
• On Friday, October 16, 1998, I moved the majority of the outline to the subsequent outline. Also added the introductory note on type-checking.