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> In some languages or implementations ASTs are available to the programmer, but this is after a tool (the compiler) has parsed the program according to the la

• lispm 9y ago
  > power to manipulate ASTs easily(with quasi-quotation)? From a point of Lisp code manipulation, quasi-quotation does not reach far and thus the convenience it provides is only shallow. It's useful to have, but its main purpose is to build up code based on some template mechanism. There are other things which are also important, like destructuring of code, which a typical macro mechanism will support in some way. Another point: to have an AST you need to have the code parsed already. The AST is an output of a parser stage. That means you are bound to the syntax this parser implements and this AST can represent - or you need to be able to feed this parser with different syntax definitions and have it runtime extensible. Since Lisp macros see only code as internal data, the input format is much less limited. It can also transform arbitrary data into code. Or you need to be able to deal in some other with code which does not conform to what the parser expects. > Irrelevant to the discussion of "code as data". Every language can read s-expr in language agnostic way. 'can' vs. usually done is a huge practical difference. s-expressions are not a standard input format for C compiler. I have not seen s-expressions or a similar data format in the C language standard used to represent code. I'd would be interested to see a reference to it, if you have one. > I never said that the languages are built on top of a neutral data format, But Lisp is. > Unless i am mistaken, the only benefit is being easy to parse by tools(editors, compilers etc) and with quasi-quotation the power of macros. Quasi-quotation is only a convenience feature - which is great, but they don't add any power to the language. Lisp can create the same effects without quasi-quotation. The benefits are: * operations on the data format can be supported in an text editor and then can be applied to both data and code * the code structure is not limited to what syntax a programming language parser implements or what an AST can represent * the manipulation of internal code as data is convenient and simple for many cases, since the typical operations can be applied * the internal format used by Lisp has a default external format, but can also be used with different external formats * the programming language implementation exposes this internal data in various stages to the user, which it then can conveniently use it. For example macros see this data and can transform it with the usual tools. Interpreters use it and can expose it to the developer. The compiler can use it as input. The pretty printer takes it as input to create textual representations. The main point is that it is convenient to use. Thus code transformations and code manipulations of s-expressions are both common and simple in Lisp. The effect is that user-written code manipulation is widely used in Lisp - even at runtime, where for example in C it is typically done in external tools in preprocessor stages or hidden from the user at compile time.
  • xfer 9y ago
    > There are other things which are also important, like destructuring of code, which a typical macro mechanism will support in some way. Destructuring doesn't require it to be a list/s-expr. You can destruct ASTs in any language that supports pattern-matching. > or you need to be able to feed this parser with different syntax definitions and have it runtime extensible. That's what languages should aim to do. For example rust's procedural macros can transform a TokenStream to another TokenStream and doesn't have to satisfy Rust language parser. Although that doesn't extend to lexing. > 'can' vs. usually done is a huge practical difference. s-expressions are not a standard input format for C compiler I am not talking about a specific language compiler, but modern languages are heading in the right direction without using s-expr. > Quasi-quotation is only a convenience feature - which is great, but they don't add any power to the language. Lisp can create the same effects without quasi-quotation. Yes, but then you write your own macro-expander. > operations on the data format can be supported in an text editor and then can be applied to both data and code Yes, but they will not be very helpful without additional tooling. > the code structure is not limited to what syntax a programming language parser implements or what an AST can represent And you have lost the previous point, now your "code as data" is not so well-supported. > the programming language implementation exposes this internal data in various stages to the user, which it then can conveniently use it. For example macros see this data and can transform it with the usual tools. Interpreters use it and can expose it to the developer. The compiler can use it as input. The pretty printer takes it as input to create textual representations. True, but these all can be provided without s-expressions, the compiler just has to be designed properly into reusable modules. Rest of your points just suggest that the tooling part is easier and simpler to do and understand, which ofcourse i agree with. But not all programmers prefer Lisp like syntax and most languages haven't really cared about it or see as undesired(macros bring some undesirable properties as languages like loop pop-up) flexibility.
    • lispm 9y ago
      > Destructuring doesn't require it to be a list/s-expr. You can destruct ASTs in any language that supports pattern-matching. It's not about requiring. It's provided in Lisp. Macros already provide destructuring in their arglists. > That's what languages should aim to do. For example rust's procedural macros can transform a TokenStream to another TokenStream and doesn't have to satisfy Rust language parser. Although that doesn't extend to lexing. A tokenizer is not a language parser. > I am not talking about a specific language compiler, but modern languages are heading in the right direction without using s-expr. That's nothing new or modern. Code transformations and code representation has been used for decades. > Yes, but then you write your own macro-expander. No, quasi-quotations have nothing to do with a macro-expander. It's a general feature in Lisp - independent of macros. One alternative is to procedurally build up code. There is no functionality difference between `(a ,a) and (list 'a a) Both compute the same list structure. >> operations on the data format can be supported in an text editor and then can be applied to both data and code > Yes, but they will not be very helpful without additional tooling. There is nothing wrong with tool building. >> the code structure is not limited to what syntax a programming language parser implements or what an AST can represent > And you have lost the previous point, now your "code as data" is not so well-supported. But I gain a lot of flexibility. > True, but these all can be provided without s-expressions, the compiler just has to be designed properly into reusable modules. That's what Lisp already provides. > Rest of your points just suggest that the tooling part is easier and simpler to do and understand, which ofcourse i agree with. But not all programmers prefer Lisp like syntax and most languages haven't really cared about it or see as undesired(macros bring some undesirable properties as languages like loop pop-up) flexibility. All true.