What Macros Are and How to Write Correct Ones Brian Goslinga - - PowerPoint PPT Presentation

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What Macros Are and How to Write Correct Ones Brian Goslinga - - PowerPoint PPT Presentation

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Introduction Macros in C Macros in Scheme Macros in Racket Conclusion What Macros Are and How to Write Correct Ones Brian Goslinga December 4, 2010 UMM Computer Science Senior Seminar Brian Goslinga What Macros Are and How to Write Correct

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Introduction Macros in C Macros in Scheme Macros in Racket Conclusion

What Macros Are and How to Write Correct Ones

Brian Goslinga December 4, 2010 UMM Computer Science Senior Seminar

Brian Goslinga What Macros Are and How to Write Correct Ones

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Introduction Macros in C Macros in Scheme Macros in Racket Conclusion

Abbreviations

English has abbreviations. Abbreviations allow for the compact representation of complex objects. Abbreviations allow us to chunk concepts together. Compare and contrast: The UMM CSci program is awesome. The University of Minnesota Morris Computer Science program is awesome.

Brian Goslinga What Macros Are and How to Write Correct Ones

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Code abbreviations

As in English, it can be useful to have abbreviations in code. for is an abbreviated while. for is built into many programming languages, but not all abbreviations are. Macros allow the programmer to add their own abbreviations to a programming language. for (int i=0;i<10;i++) { doStuff(i); } int i = 0; while (i < 10) { doStuff(i); i++; }

Brian Goslinga What Macros Are and How to Write Correct Ones

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Outline

The outline for the rest of the talk: Macros and What Can Go Wrong Macros in the C programming language Macros in the Scheme programming language Macros in the Racket programming language Conclusions

Brian Goslinga What Macros Are and How to Write Correct Ones

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The Essence of Macros

Like abbreviations, macros perform source-to-source transformations on the program. The compiler macro expands source code by running all macros that appear in it. The macro expansion of a macro is what a macro stands for.

Brian Goslinga What Macros Are and How to Write Correct Ones

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A Simple Example

unless is the same as if, but with a negated conditional. unless <test>: <body> → if not <test>: <body> Before: unless fileEmpty(f): readData(f) After: if not fileEmpty(f): readData(f) In most languages, unless cannot be a function because functions evaluate their arguments before they are called. (Call-by-value) unless(fileEmpty(f), readData(f)) would attempt to read from f even if it was empty.

Brian Goslinga What Macros Are and How to Write Correct Ones

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What Can Go Wrong

The macro expansion is used in place of the macro. The expansion must behave correctly in that context. Variables names may be duplicated, or code may be evaluated too many times. Both can lead to hard-to-find errors. The first is especially bad as it only occurs when the programmer picks exactly the wrong name for a variable.

Brian Goslinga What Macros Are and How to Write Correct Ones

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The Concept of Hygiene

A macro is hygienic if its macro expansion does not cause these types of errors. The ability to write hygienic macros in a language is crucial if macro are to be a reliable part of the language. Can be done by hand, but some languages have a hygienic macro system, making macros automatically be hygienic.

Brian Goslinga What Macros Are and How to Write Correct Ones

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Macros in C

C has a unhygienic macros based on text substitution. #define SQ(x) x*x SQ(2+3) → 2+3*2+3 → 11 Parentheses help: #define SQ(x) ((x)*(x)) SQ(2+3) → ((2+3)*(2+3)) → 25

Brian Goslinga What Macros Are and How to Write Correct Ones

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Macros in C (Cont.)

Double evaluation is an issue: #define SQ(x) ((x)*(x)) int a = 4; SQ(++a) → ((++a)*(++a)) a will become 6. Variables clash: #define SWAP(x,y) {int tmp;tmp=x;x=y;y=tmp;} int tmp = 5, val = 3; SWAP(tmp,val) → {int tmp;tmp=tmp;tmp=val;val=tmp;} SWAP does not swap the variables.

Brian Goslinga What Macros Are and How to Write Correct Ones

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Introduction Macros in C Macros in Scheme Macros in Racket Conclusion

Macros in C (Cont.)

Double evaluation is an issue: #define SQ(x) ((x)*(x)) int a = 4; SQ(++a) → ((++a)*(++a)) a will become 6. Variables clash: #define SWAP(x,y) {int tmp;tmp=x;x=y;y=tmp;} int tmp = 5, val = 3; SWAP(tmp,val) → {int tmp;tmp=tmp;tmp=val;val=tmp;} SWAP does not swap the variables.

Brian Goslinga What Macros Are and How to Write Correct Ones

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Introduction Macros in C Macros in Scheme Macros in Racket Conclusion

Macros in C (Cont.)

Double evaluation is an issue: #define SQ(x) ((x)*(x)) int a = 4; SQ(++a) → ((++a)*(++a)) a will become 6. Variables clash: #define SWAP(x,y) {int tmp;tmp=x;x=y;y=tmp;} int tmp = 5, val = 3; SWAP(tmp,val) → {int tmp;tmp=tmp;tmp=val;val=tmp;} SWAP does not swap the variables.

Brian Goslinga What Macros Are and How to Write Correct Ones

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The Lisp Family of Programming Languages

Includes Common Lisp, Scheme, and Racket (a dialect of Scheme, formerly PLT/Dr Scheme). They are functional languages, as compared to imperative languages such as Java and C. They are well-known for their macros. Scheme and Racket have hygienic macro systems, Common Lisp has facilities for manually adding hygiene. Source code is comprised of lists. Prefix notation is used (the function is first). 2 · 3 + 1 is (+ (* 2 3) 1)

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syntax-rules

Macros are defined in Scheme using syntax-rules. It defines hygienic macros using pattern matching. We will consider a simplified subset of syntax-rules: (syntax-rules (rule template)+). Rules are tried in order. Once a successful match is found, the template is filled in and becomes the macro expansion.

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syntax-rules Example

Suppose we wanted a -> macro such that: (-> 2 (* 3) (+ 1)) (define-syntax -> (syntax-rules ((-> form (f args ...)) (f form args ...)) ((-> form next-form forms ...) (-> (-> form next-form) forms ...))))

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syntax-rules Example

Suppose we wanted a -> macro such that: (-> 2 (* 3) (+ 1)) → (-> (* 2 3) (+ 1)) (define-syntax -> (syntax-rules ((-> form (f args ...)) (f form args ...)) ((-> form next-form forms ...) (-> (-> form next-form) forms ...))))

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syntax-rules Example

Suppose we wanted a -> macro such that: (-> 2 (* 3) (+ 1)) → (-> 6 (+ 1)) (define-syntax -> (syntax-rules ((-> form (f args ...)) (f form args ...)) ((-> form next-form forms ...) (-> (-> form next-form) forms ...))))

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syntax-rules Example

Suppose we wanted a -> macro such that: (-> 2 (* 3) (+ 1)) → (+ 6 1) (define-syntax -> (syntax-rules ((-> form (f args ...)) (f form args ...)) ((-> form next-form forms ...) (-> (-> form next-form) forms ...))))

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Introduction Macros in C Macros in Scheme Macros in Racket Conclusion

syntax-rules Example

Suppose we wanted a -> macro such that: (-> 2 (* 3) (+ 1)) → 7 (define-syntax -> (syntax-rules ((-> form (f args ...)) (f form args ...)) ((-> form next-form forms ...) (-> (-> form next-form) forms ...))))

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Expanding a syntax-rules Macro

Example: (-> 2 (* 3) (+ 1)) Rule: (-> form (f args ...)) Template: (f form args ...) (define-syntax -> (syntax-rules ((-> form (f args ...)) (f form args ...)) ((-> form next-form forms ...) (-> (-> form next-form) forms ...))))

Brian Goslinga What Macros Are and How to Write Correct Ones

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Introduction Macros in C Macros in Scheme Macros in Racket Conclusion

Expanding a syntax-rules Macro

Example: (-> 2 (* 3) (+ 1)) Rule: (-> form next-form forms ...) Template: (-> (-> form next-form) forms ...) (define-syntax -> (syntax-rules ((-> form (f args ...)) (f form args ...)) ((-> form next-form forms ...) (-> (-> form next-form) forms ...))))

Brian Goslinga What Macros Are and How to Write Correct Ones

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Introduction Macros in C Macros in Scheme Macros in Racket Conclusion

Expanding a syntax-rules Macro

Example: (-> 2 (* 3) (+ 1)) Rule: (-> form next-form forms ...) Template: (-> (-> form next-form) forms ...) (define-syntax -> (syntax-rules ((-> form (f args ...)) (f form args ...)) ((-> form next-form forms ...) (-> (-> form next-form) forms ...))))

Brian Goslinga What Macros Are and How to Write Correct Ones

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Introduction Macros in C Macros in Scheme Macros in Racket Conclusion

Expanding a syntax-rules Macro

Example: (-> 2 (* 3) (+ 1)) Rule: (-> form next-form forms ...) Template: (-> (-> 2 next-form) forms ...) (define-syntax -> (syntax-rules ((-> form (f args ...)) (f form args ...)) ((-> form next-form forms ...) (-> (-> form next-form) forms ...))))

Brian Goslinga What Macros Are and How to Write Correct Ones

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Introduction Macros in C Macros in Scheme Macros in Racket Conclusion

Expanding a syntax-rules Macro

Example: (-> 2 (* 3) (+ 1)) Rule: (-> form next-form forms ...) Template: (-> (-> 2 (* 3)) forms ...) (define-syntax -> (syntax-rules ((-> form (f args ...)) (f form args ...)) ((-> form next-form forms ...) (-> (-> form next-form) forms ...))))

Brian Goslinga What Macros Are and How to Write Correct Ones

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Introduction Macros in C Macros in Scheme Macros in Racket Conclusion

Expanding a syntax-rules Macro

Example: (-> 2 (* 3) (+ 1)) Rule: (-> form next-form forms ...) Template: (-> (-> 2 (* 3)) (+ 1)) (define-syntax -> (syntax-rules ((-> form (f args ...)) (f form args ...)) ((-> form next-form forms ...) (-> (-> form next-form) forms ...))))

Brian Goslinga What Macros Are and How to Write Correct Ones

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Introduction Macros in C Macros in Scheme Macros in Racket Conclusion

Expanding a syntax-rules Macro

Example: (-> 2 (* 3) (+ 1)) Rule: (-> form next-form forms ...) Template: (-> (-> 2 (* 3)) (+ 1)) (define-syntax -> (syntax-rules ((-> form (f args ...)) (f form args ...)) ((-> form next-form forms ...) (-> (-> form next-form) forms ...))))

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Expanding a syntax-rules Macro

(-> (-> 2 (* 3)) (+ 1)) Expands to (+ (-> 2 (* 3)) 1) Expands to (+ (* 2 3) 1) (define-syntax -> (syntax-rules ((-> form (f args ...)) (f form args ...)) ((-> form next-form forms ...) (-> (-> form next-form) forms ...))))

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Achievements And Room For Improvement

syntax-rules defines hygienic macros, and the macros look like what they produce. syntax-rules has stood the test of almost 20 years of use. syntax-rules is unable to notice a large range of syntax errors. syntax-rules does not provide good feedback on bad syntax.

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syntax-parse

Introduced by Northeastern researchers Ryan Culpepper and Matthias Felleisen in 2010. Has been in Racket for about a year. Has more advanced patterns; pattern variables are tagged with a syntax class, such as a list, a number, etc. Allows for user-defined syntax classes. Automatically generates good error messages from the descriptions

  • f syntax classes.

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let As a Macro

lambda is the keyword in Scheme and Racket for introducing an anonymous function. (lambda (x y) (+ x y)) let is a way to introduce local variables in Scheme and Racket. (let ((x 3) (y 2)) (+ x y)) let can be written in terms of lambda: (let ((x 3) (y 2)) (+ x y)) → ((lambda (x y) (+ x y)) 3 2)

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let As a Macro

When defined using syntax-rules (let ((x 1) (x 2)) (h x)) lambda: duplicate argument name in: x (let (((x y) (f 7))) (g x y)) reference to an identifier before its definition: y (let (x 5) (add1 x)) let: bad syntax in: (let (x 5) (add1 x)) (let 17) let: bad syntax in: (let 17)

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let As a Macro

When defined using syntax-parse (let ((x 1) (x 2)) (h x)) let: duplicate variable name in: x (let (((x y) (f 7))) (g x y)) let: expected identifier in: (x y) (let (x 5) (add1 x)) let: expected binding pair in: x (let 17) let: expected sequence of binding pairs in: 17

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Conclusions

Macros allow the programmer to extend a programming language with additional constructs. A macro must be hygienic for it to behave correctly. Hygienic macro systems give hygiene for free. syntax-parse allows macros to produce good error messages.

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References

  • W. Clinger and J. Rees. Macros that work. POPL ’91. 1991
  • E. Kohlbecker, D. P. Friedman, M. Felleisen, B. Duba. Hygienic

macro expansion. LFP ’86. 1986

  • E. E. Kohlbecker, M. Wand. Macro-by-example. POPL ’87. 1987
  • R. Culpepper, M. Felleisen. Fortifying macros. ICFP ’10. 2010

Brian Goslinga What Macros Are and How to Write Correct Ones