Comment by lispm | Hacker News Reader

Comment by lispm | Hacker News Reader

I don't think programs are usually strings.

Most programs I see are text files. If I edit them in an editor, I don't know what data structure represents them, but it might be a doubly linked list of strings, or some other data structure.

If a compiler reads a program, it uses typically a stream of characters.

A string would be something like this: a delimiter, some characters, the delimiter. Example: "a + b". It could also be a string data object in some program.

But programs are not always represented as strings and many are not. Sometimes they are. For example an EVAL procedure may take a string as an argument - then code is represented as a string or passed at runtime as a string object.

When we say in Lisp code is data, we actually mean code is data, but not in the trivial form of text representation formats (text files, strings, ...).

It's an abbreviation. We expect usually not to mention, that it is not of the trivial version. Like when we speak of a picture with lots of color, we don't mention that this excludes pictures with only black, white and shades of grey.

Generally, if I see

  a + b

then it says nothing about a data structure and its syntax to represent that. All I know that it must be a syntactical legal expression in some programming language.

If I see Lisp code in the form of

  (+ a b)

I know that this uses the data syntax for s-expressions to represent that code.

I then would require that this is syntactical legal Lisp - THIS requires that in an s-expression, the + operator comes first - that it uses prefix syntax.

Wien we mean code as data, we mean that Lisp uaes an actual and explicit data format to encode its programs: s-expressions. The data format is similar to JSON or XML, in that it has an explicit external representation. It's also similar to JSON, because both have an internal representation in a programming language.

If we would use JSON as a format to encode programs, we would also be able to say, code is data.

S-expressions are slightly more effective as data encoding for programs than JSON, because it uses less delimiters and encodes a rich set of data structures.

So, yes. Code with a non-trivial and simple data-representation makes Lisp cool.

> Wien we mean code as data, we mean that Lisp uaes an actual and explicit data format to encode its programs: s-expressions.

Every programming language uses an "actual" and explicit data format to encode its programs, the valid programs all parse to the corresponding AST.

The only thing unique about Lisp here is a construct to parse the serialization format(s-expressions) and perform actions on it i.e. macros. The fact that parsing s-expressions is so trivial makes it seem like manipulating code as data is somehow unique to Lisp.

> Every programming language uses an "actual" and explicit data format to encode its programs

where? Show me the data format which is used by C programmers to encode programs.

A data format with an external and internal representation, and both used by the programmer.

An AST of a C compiler isn't it, since the programmer isn't using it. Neither externally as text, nor internally as data. It's only used internally by a compiler - if it does use one. The C programmer does not write code with ASTs, nor does he even manipulate code via ASTs operations.

It's not that some languages have ASTs -> they are simply no neutral data format and the programmer isn't using this to write/read programs. 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 language syntax rules. But that is not what Lisp does, s-expressions are more like a general data format which by the Lisp reader (a tool) is used to translate into a token tree. But the Lisp reader is not a compiler internal stage. It's a general tool, which turns externalized data into internalized data.

ASTs are also a different thing from s-expressions, since s-expressions don't represent abstract syntax of Lisp programs. S-expressions are language agnostic: it does know nothing about the syntax of Lisp constructs and doesn't represent no syntactic information. On the s-expression level (a b c) is no different from (+ 1 2) or (defun foo ()).

The C syntax isn't layered on top of a neutral data format, like Lisp is.

> 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 language syntax rules. But that is not what Lisp does, s-expressions are more like a general data format which by the Lisp reader (a tool) is used to translate into a token tree. But the Lisp reader is not a compiler internal stage. It's a general tool, which turns externalized data into internalized data.

So, what additional power Lisp has as "code as data" vs the language that gives power to manipulate ASTs easily(with quasi-quotation)?

>ASTs are also a different thing from s-expressions, since s-expressions don't represent abstract syntax of Lisp programs. S-expressions are language agnostic: it does know nothing about the syntax of Lisp constructs and doesn't represent no syntactic information.

Irrelevant to the discussion of "code as data". Every language can read s-expr in language agnostic way.

> The C syntax isn't layered on top of a neutral data format, like Lisp is.

I never said that the languages are built on top of a neutral data format, but asking the benefits of being so. 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.

> 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.

> 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.

> 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.

> A tokenizer is not a language parser.

It's not a tokenizer it can transform a token stream to another token stream with a given function which can be some language parser/just rust code producing desired rust ast.

> No, quasi-quotations have nothing to do with a macro-expander.

My bad, i was confusing it with mixing evaluation.

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