Allow parsing of "cond && var=value" and similar expressions #170
Conversation
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Expressions like "1 && x=1" aren't really valid (IMO), because assignments are lower-precedence than binary operators, but onetrueawk, Gawk, and mawk all support this for logical, match and comparison operators. The other awks support this by using a yacc grammar which supports backtracking, and as Vitus13 said on reddit: "If there are two syntactically valid parsings and one is a semantic error, the error handling may resolve the ambiguity towards the valid parsing. In this case, you can only assign to L values, so trying to assign to (1&&x) doesn't make any sense." In GoAWK, this requires a form of backtracking (I call it "partial backtracking" because it's not actually backing up the lexer). It works by parsing as (1&&x)=1 according to the operator precedence, then determining that you're trying to assign something that isn't an lvalue, then confirming that 1&&x is a binary expression, that the "x" part is an lvalue, and that the operator (&& in this case) is one that the other awks handle similarly for this case. Also make the error message a bit clearer when you don't have an lvalue on the left hand side of an assignment, like "rand() = 1". Fixes #166
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
Expressions like "1 && x=1" aren't really valid (IMO), because assignments are lower-precedence than binary operators, but onetrueawk, Gawk, and mawk all support this for logical, match and comparison operators.
The other awks support this by using a yacc grammar which supports backtracking, and as Vitus13 said on reddit: "If there are two syntactically valid parsings and one is a semantic error, the error handling may resolve the ambiguity towards the valid parsing. In this case, you can only assign to L values, so trying to assign to (1&&x) doesn't make any sense."
In GoAWK, this requires a form of backtracking (I call it "partial backtracking" because it's not actually backing up the lexer). It works by parsing as (1&&x)=1 according to the operator precedence, then determining that you're trying to assign something that isn't an lvalue, then confirming that 1&&x is a binary expression, that the "x" part is an lvalue, and that the operator (&& in this case) is one that the other awks handle similarly for this case.
Also make the error message a bit clearer when you don't have an lvalue on the left hand side of an assignment, like "rand() = 1".
Fixes #166