lexer | Base library for a lexer that can be used in Top-Down, Recursive Descent Parsers | Parser library

Your second approach can work, but in a roundabout way, as mikefarah/yq does not support updating multi-line block literals yet

One way to solve this, with the existing constructs would be to do below, without having to create a temporary YAML file

Meditating on Tomas' answer, I have come up with the following that works

The code of lexer_lexbuf_to_supplier is as follows:

I think you'll agree that most languages (likely including the one you are implementing) have conceptual tokens:

• operators, e.g * (usually multiply), '(', ')', ;
• keywords, e.g., "IF", "GOTO"
• identifiers, e.g. FOO, count, ...
• numbers, e.g. 0, -527.23E-41
• comments, e.g., /* this text is ignored in your file */
• whitespace, e.g., sequences of blanks, tabs and newlines, that are ignored

As a practical matter, it takes a specific chunk of code to scan for/collect the characters that make each individual token. You'll need such a code chunk for each type of token your language has.

If you write a parser without a lexer, at each point where your parser is trying to decide what comes next, you'll have to have ALL the code that recognize the tokens that might occur at that point in the parse. At the next parser point, you'll need all the code to recognize the tokens that are possible there. This gives you an immense amount of code duplication; how many times do you want the code for blanks to occur in your parser?

If you think that's not a good way, the obvious cure to is remove all the duplication: place the code for each token in a subroutine for that token, and at each parser place, call the subroutines for the tokens. At this point, in some sense, you already have a lexer: an isolated collection of code to recognize tokens. You can code perfectly fine recursive descent parsers this way.

The next thing you'll discover is that you call the token subroutines for many of the tokens at each parser point. Even that seems like a lot of work and duplication. So, replace all the calls with a single "GetNextToken" call, that itself invokes the token recognizing code for all tokens, and returns a enum that identifies the specific token encountered. Now your parser starts to look reasonable: at each parser point, it makes one call on GetNextToken, and then branches on enum returned. This is basically the interface that people have standardized on as a "lexer".

One thing you will discover is the token-lexers sometimes have trouble with overlaps; keywords and identifiers usually have this trouble. It is actually easier to merge all the token recognizers into a single finite state machine, which can then distinguish the tokens more easily. This also turns out to be spectacularly fast when processing the programming language source text. Your toy language may never parse more than 100 lines, but real compilers process millions of lines of code a day, and most of that time is spent doing token recognition ("lexing") esp. white space suppression.

You can code this state machine by hand. This isn't hard, but it is rather tedious. Or, you can use a tool like FLEX to do it for you, that's just a matter of convenience. As the number of different kinds of tokens in your language grows, the FLEX solution gets more and more attractive.

TLDR: Your parser is easier to write, and less bulky, if you use a lexer. In addition, if you compile the individual lexemes into a state machine (by hand or using a "lexer generator"), it will run faster and that's important.

In that case you should postpone parsing. You can for example make a TNumByte data constructor that stores the value as String:

The problem is not the order of interpretation, which is top to bottom as you expect; it's the scope. In Python, when referencing a global variable in a narrower function scope, if you modify the value, you must first tell the code that the global variable is the variable you are referencing, instead of a new local one. You do this with the global keyword. In this example, your program should actually look like this:

first: No the article does not says it allows initializer in catch statement.

It says to refer to the function specification for

• type-specifier-seq
• declarator
• abstract-declarator

or the initializer is a different element.

Second:

An initializer in a catch statement is pointless since a value is required for actualy going into a catch statement. which means that the initializer value would never be used, and it would jeopardize the program flow since the initalization could raise an exception that will at best terminate.

MSVC probably tolerate it out of pointlessness since it cannot affect the program

That problem is solved by declaring

Your question boils down to: "how can I convert my parse tree to an abstract syntax tree?". The simple answer to that is: "you can't" :). At least, not using a built-in ANTLR mechanism. You'll have to traverse the parse tree (using ANTLR's visitor- or listener mechanism) and construct your AST manually.

The feature to more easily create AST's from a parse tree often pops up both in ANTLR's Github repo:

• https://github.com/antlr/antlr4/issues/2428

as well as on stackoverflow:

• Generate AST using ANTLR4 generated visitor
• ANTLR4 AST Creation - How to create an AstVistor