VII. Error Checking and Diagnostics
Reading edition
In a complete compilation the compiler makes two passes on the Algol source program. If errors which the compiler cannot correct are detected in the first pass, then the second, or translation, pass will not be made. The following types of errors are detected:
1. syntactical error
2. undeclared identifier
3. identifier declared twice in the same block head
4. misspelled delimiter (corrected in many cases)
5. missing escape symbol (corrected unless both are missing for the same delimiter, in which case the delimiter is treated as an identifier).
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The program listing and any diagnostics always appear on the standard output medium. In the case of a syntactical error, a message will appear in the program listing one or several lines below the error. The location of the error in the program will be further pinpointed in the line of symbols immediately below the error message. This line will be a short portion of the program with the last symbol in the line being the one which indicates the error. For example, a declaration might be out of place as follows:
. . . x := a + b; 'INTEGER' K;
**** LAST CHARACTER INDICATES SYNTACTICAL ERROR.
x := a + b; INTEGER . . .
In some cases the line below the message may differ slightly from the corresponding string of symbols above; for example, an identifier might be rendered by Ident. It is possible for a single syntactical error to cause more than one diagnostic.
A few syntactical errors are corrected by the compiler, and a message is put out to this effect. An example is a semicolon immediately preceding else.
According to the comment conventions of Algol, any string of symbols following end and not containing end, else or a semicolon is 19 treated as comment. As a result, the omission of one of these symbols following end does not always cause an error in compilation but will cause a portion of the program to be skipped over by the compiler. Thus for example, in
... x := a + b end for i := 1 step 1 ...
the FOR statement will be skipped at least in part. The compiler will put out a caution message in this and some other cases, but it will not change the program.
If an identifier is not declared (or possibly declared in the wrong place), a message is put out below the program listing together with the undeclared identifier.
The compiler does not check the type of identifiers. Therefore, such errors as a Boolean variable in an arithmetic expression, or the brackets of a subscripted variable replaced by parentheses, are not detected, and an incorrect program may be compiled.