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Lisp has reader macros which allow you to reprogram its lexer. Lisp macros allow you to program the translation from the visible structure to the parse tree. F

by fanf2 2y ago

Lisp has reader macros which allow you to reprogram its lexer. Lisp macros allow you to program the translation from the visible structure to the parse tree. For example, https://pyret.org/ https://pyret.org/ It really isn’t simple or necessarily uniform.

__MatrixMan__ 2y ago
I've heard that certain lisps (Common Lisp comes up when I search for reader macros) allow for all kinds of tinkering with themselves. But the ability of one to make itself not a lisp anymore, while interesting, doesn't seem to say much about the merits of sticking to s-expressions, except maybe to point out that somebody once decided not to.
- _19qg 2y ago
  Reader macros are there to program and configure the reader. The reader is responsible for reading s-expressions into internal data structures. There are basically two main uses of reader-macros: data structures and reader control. A CL implementation will implement reading lists, symbols, numbers, arrays, strings, structures, characters, pathnames, ... via reader macros. Additionally the reader implements various forms of control operations: conditional reading, reading and evaluation, circular datastructures, quoting and comments. This is user programmable&configurable. Most uses will be in the two above categories: data structure syntax and control. For example we could add a syntax for hash tables to s-expressions. An example for a control extension would be to add support for named readtables. For example a Common Lisp implementation could add a readtable for reading s-expressions from Scheme, which has a slightly different syntax. Reader macros were optimized for implementing s-expressions, thus the mechanism isn't that convenient as a lexer/parser for actual programming languages. It's a a bit painful to do so, but possible. A typical reader macro usage, beyond the usage described above, is one which implements a different token or expression syntax. For example there are reader macros which parse infix expressions. This might be useful in Lisp code where arithmetic expressions can be written in a more conventional infix syntax. The infix reader macro would convert infix expressions into prefix data.
lispm 2y ago
Is Pyret based on reader macros? I would think it's much easier to use a syntax parser for that.

by fanf2 2y ago

__MatrixMan__ 2y ago

I've heard that certain lisps (Common Lisp comes up when I search for reader macros) allow for all kinds of tinkering with themselves. But the ability of one to make itself not a lisp anymore, while interesting, doesn't seem to say much about the merits of sticking to s-expressions, except maybe to point out that somebody once decided not to.

_19qg 2y ago
Reader macros are there to program and configure the reader. The reader is responsible for reading s-expressions into internal data structures. There are basically two main uses of reader-macros: data structures and reader control. A CL implementation will implement reading lists, symbols, numbers, arrays, strings, structures, characters, pathnames, ... via reader macros. Additionally the reader implements various forms of control operations: conditional reading, reading and evaluation, circular datastructures, quoting and comments. This is user programmable&configurable. Most uses will be in the two above categories: data structure syntax and control. For example we could add a syntax for hash tables to s-expressions. An example for a control extension would be to add support for named readtables. For example a Common Lisp implementation could add a readtable for reading s-expressions from Scheme, which has a slightly different syntax. Reader macros were optimized for implementing s-expressions, thus the mechanism isn't that convenient as a lexer/parser for actual programming languages. It's a a bit painful to do so, but possible. A typical reader macro usage, beyond the usage described above, is one which implements a different token or expression syntax. For example there are reader macros which parse infix expressions. This might be useful in Lisp code where arithmetic expressions can be written in a more conventional infix syntax. The infix reader macro would convert infix expressions into prefix data.

lispm 2y ago

Is Pyret based on reader macros? I would think it's much easier to use a syntax parser for that.