A couple of days ago I promised to get back to this topic (if you want to refresh your memory, here is the discussion of those operators in Qi). Today we will discuss Karma’s unary operators ‘!’ and ‘~’. These have very similar semantics as their counterparts in Qi, but as usual, we have to turn things inside out in order to make them fit to output generation.
Starting with Spirit V2 we added a module for generating code aimed at the lexical analysis of the input: Spirit.Lex (a lexer module, also called scanner). Lexical analysis is the process of preprocessing the stream of input characters and separating it into strings called tokens, most of the time delimited by whitespace. Most compiler texts start here, and devote several chapters to discussing various ways to build scanners. Spirit.Lex is a library built to take care of the complexities of creating a lexer for your grammar.
We know the documentation of Spirit.Lex is not complete yet. So I will write more about it here from now on to fill in the missing pieces and to show a couple of tricks demonstrating its best usage.
In the previous installment of the ‘Tip of the Day’ I started to talk about some lesser known features related to semantic actions. Today I will highlight some more details. If a semantic action is attached to a component which is part of an expression assigned to a rule (the rule’s right hand side) it is not only possible to access the attributes of the components it is connected with. In addition it is possible to access rule specific values! Sounds interesting? Read on!
The concept of semantic actions seems to be quite easy to understand. It appears to be at least easier to grasp than the concept of attribute propagation. This might be because semantic actions have been part of Spirit for almost a decade now. Additionally, with semantic actions data flow control is tightly connected to the component the semantic action is attached to, so the effect is highly localized and easy to spot.
Spirit has some new features related to semantics actions. That’s reason enough to talk about how attributes can be accessed from inside semantic actions.
The Freenet #boost IRC channel amazes me every day with the amount of interest Spirit is getting from a lot of people. Thanks to everyone over there! But the best is those people are asking many interesting questions allowing me to come up with yet another Tip of the Day.
Today’s question has been asked by @psicode: “What is the difference between the components created by the unary operators ‘!’ and ‘~’?”. As the semantics of those operators are slightly dissimilar in Qi and Karma, I will talk about them separately. I will write about the Qi operators today and about the corresponding Karma operators in one of the next installments.
If you read the article about attribute handling for non-terminals (The Magical Power of Attributes in Spirit – Directives and Non-terminals) you might remember that Spirit’s non-terminals (rules and grammars) are somewhat special with regard to their attribute handling. In today’s ‘Tip of the Day’ I would like to revisit this topic as it still seems to be difficult to understand.
Over at the Freenet #boost IRC channel somebody (I think it was @VeXocide) suggested to write a ‘Tip of the Day’ about the Qi directive raw[]. I was told this ‘was a major stumbling stone’ while learning Qi. I always appreciate to get suggestions for articles, so here we go…
While looking through the mailing list archives I realized that often the small issues get into our way. Small snippets of information are making the difference. So I decided to start a (more or less regular) series about small tips helping to get your work done while using Spirit. Even if I sloppily call this series ‘Tip of the Day’, I by no means plan to have a tip a day.
This time I’m going to highlight the difference between three different ways to parse or generate a single character: ‘a’, lit(‘a’), and char_(‘a’).
Recent Comments