From a2fc5a4dbaeb00764f27b38cbc622c10ad4ffbf9 Mon Sep 17 00:00:00 2001 From: Greg Hendershott Date: Fri, 26 Oct 2012 11:53:30 -0400 Subject: [PATCH] Regen index.html for latest changes. --- index.html | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/index.html b/index.html index 1223d24..5fa7572 100644 --- a/index.html +++ b/index.html @@ -2,7 +2,7 @@ Fear of Macros
►Fear of Macros
1 Introduction
2 The plan of attack
3 Transformers
3.1 What is a syntax transformer?
3.2 What’s the input, Kenneth?
3.3 Actually transforming the input
3.4 Compile time vs. run time
4 Pattern matching: syntax-case and syntax-rules
4.1 Patterns and templates
4.1.1 "A pattern variable can’t be used outside of a template"
5 Syntax parameters
6 Robust macros: syntax-parse
7 Other questions
7.1 What’s the point of with-syntax?
7.2 What’s the point of begin-for-syntax?
7.3 What’s the point of racket/ splicing?
8 References/ Acknowledgments
9 Epilogue

Fear of Macros

Greg Hendershott

Copyright (c) 2012 by Greg Hendershott. All rights reserved. -
Last updated 2012-10-26 08:58:25

    1 Introduction

    2 The plan of attack

    3 Transformers

      3.1 What is a syntax transformer?

      3.2 What’s the input, Kenneth?

      3.3 Actually transforming the input

      3.4 Compile time vs. run time

    4 Pattern matching: syntax-case and syntax-rules

      4.1 Patterns and templates

        4.1.1 "A pattern variable can’t be used outside of a template"

    5 Syntax parameters

    6 Robust macros: syntax-parse

    7 Other questions

      7.1 What’s the point of with-syntax?

      7.2 What’s the point of begin-for-syntax?

      7.3 What’s the point of racket/splicing?

    8 References/Acknowledgments

    9 Epilogue

1 Introduction

I learned Racket after 25 years of doing C/C++ imperative programming.

Some psychic whiplash resulted.

"All the parentheses" was actually not a big deal. Instead, the first +

Last updated 2012-10-26 11:45:11

    1 Introduction

    2 The plan of attack

    3 Transformers

      3.1 What is a syntax transformer?

      3.2 What’s the input, Kenneth?

      3.3 Actually transforming the input

      3.4 Compile time vs. run time

    4 Pattern matching: syntax-case and syntax-rules

      4.1 Patterns and templates

        4.1.1 "A pattern variable can’t be used outside of a template"

    5 Syntax parameters

    6 Robust macros: syntax-parse

    7 Other questions

      7.1 What’s the point of with-syntax?

      7.2 What’s the point of begin-for-syntax?

      7.3 What’s the point of racket/splicing?

    8 References/Acknowledgments

    9 Epilogue

1 Introduction

I learned Racket after 25 years of doing C/C++ imperative programming.

Some psychic whiplash resulted.

"All the parentheses" was actually not a big deal. Instead, the first mind warp was functional programming. Before long I wrapped my brain around it, and went on to become comfortable and effective with many other aspects and features of Racket.

But two final frontiers remained: Macros and continuations.

I found that simple macros were easy and understandable, plus there @@ -52,7 +52,7 @@ always outputs syntax for a string literal:

foo, please give it to my transformer function, and replace it with the syntax I give back to you." So Racket will give anything that looks like (foo ...) to our function, and we can change it. Much like a -search-and-replace.

Maybe you know that the usual way to define a function in Racket:

(define (f x) ...)

is shorthand for:

(define f (lambda (x) ...))

That shorthand lets you avoid typing lambda and some parentheses.

Well there is a similar shorthand for define-syntax:

> (define-syntax (also-foo stx)
    #'"I am also foo")
> (also-foo)

"I am also foo"

What we want to remember is that this is simply shorthand. We are +search-and-replace.

Maybe you know that the usual way to define a function in Racket:

(define (f x) ...)

is shorthand for:

(define f (lambda (x) ...))

That shorthand lets you avoid typing lambda and some parentheses.

Well there is a similar shorthand for define-syntax:

> (define-syntax (also-foo stx)
    #'"I am also foo")
> (also-foo)

"I am also foo"

What we want to remember is that this is simply shorthand. We are still defining a transformer function, which takes syntax and returns syntax. Everything we do with macros, will be built on top of this basic idea. It’s not magic.

Speaking of shorthand, there is also a shorthand for syntax, @@ -71,7 +71,7 @@ information about lexical scoping (which you don’t need to worry about now, but will turn out to be important later.)

There are a variety of functions available to access a syntax object:

> (define stx #'(if x (list "true") #f))
> (syntax->datum stx)

'(if x (list "true") #f)

> (syntax-e stx)

'(#<syntax:11:0 if> #<syntax:11:0 x> #<syntax:11:0 (list "true")> #<syntax:11:0 #f>)

> (syntax->list stx)

'(#<syntax:11:0 if> #<syntax:11:0 x> #<syntax:11:0 (list "true")> #<syntax:11:0 #f>)

> (syntax-source stx)

'eval

> (syntax-line stx)

11

> (syntax-column stx)

0

When we want to transform syntax, we’ll generally take the pieces we were given, maybe rearrange their order, perhaps change some of the pieces, and often introduce brand-new pieces.

3.3 Actually transforming the input

Let’s write a transformer function that reverses the syntax it was -given:

> (define-syntax (reverse-me stx)
    (datum->syntax stx (reverse (cdr (syntax->datum stx)))))
> (reverse-me "backwards" "am" "i" values)

"i"

"am"

"backwards"

Understand Yoda, can we. Great, but how does this work?

First we take the input syntax, and give it to +given:

> (define-syntax (reverse-me stx)
  (datum->syntax stx (reverse (cdr (syntax->datum stx)))))
> (reverse-me "backwards" "am" "i" values)

"i"

"am"

"backwards"

Understand Yoda, can we. Great, but how does this work?

First we take the input syntax, and give it to syntax->datum. This converts the syntax into a plain old list:

> (syntax->datum #'(reverse-me "backwards" "am" "i" values))

'(reverse-me "backwards" "am" "i" values)

Using cdr slices off the first item of the list, reverse-me, leaving the remainder: