►Syntax:   Meta-Programming Helpers
▼1 Parsing and Specifying Syntax
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1.1 Introduction

This section provides an introduction to writing robust macros with syntax-parse and syntax classes.

As a running example we use the following task: write a macro named mylet that has the same syntax and behavior as Racket’s let form. The macro should produce good error messages when used incorrectly.

Here is the specification of mylet’s syntax:

(mylet ([var-id rhs-expr] ...) body ...+)
(mylet loop-id ([var-id rhs-expr] ...) body ...+)

For simplicity, we handle only the first case for now. We return to the second case later in the introduction.

The macro can be implemented very simply using define-syntax-rule:

> (define-syntax-rule (mylet ([var rhs] ...) body ...)
    ((lambda (var ...) body ...) rhs ...))

When used correctly, the macro works, but it behaves very badly in the presence of errors. In some cases, the macro merely fails with an uninformative error message; in others, it blithely accepts illegal syntax and passes it along to lambda, with strange consequences:

> (mylet ([a 1] [b 2]) (+ a b))

3

> (mylet (b 2) (sub1 b))

mylet: use does not match pattern: (mylet ((var rhs) ...)

body ...) at: (mylet (b 2) (sub1 b))

> (mylet ([1 a]) (add1 a))

lambda: not an identifier, identifier with default, or

keyword at: 1

> (mylet ([#:x 1] [y 2]) (* x y))

eval:2:0: arity mismatch;

 the expected number of arguments does not match the given

number

  expected: 0 plus an argument with keyword #:x

  given: 2

  arguments...:

   1

   2

These examples of illegal syntax are not to suggest that a typical programmer would make such mistakes attempting to use mylet. At least, not often, not after an initial learning curve. But macros are also used by inexpert programmers and as targets of other macros (or code generators), and many macros are far more complex than mylet. Macros must validate their syntax and report appropriate errors. Furthermore, the macro writer benefits from the machine-checked specification of syntax in the form of more readable, maintainable code.

We can improve the error behavior of the macro by using syntax-parse. First, we import syntax-parse into the transformer environment, since we will use it to implement a macro transformer.

> (require (for-syntax syntax/parse))

The following is the syntax specification above transliterated into a syntax-parse macro definition. It behaves no better than the version using define-syntax-rule above.

> (define-syntax (mylet stx)
    (syntax-parse stx
      [(_ ([var-id rhs-expr] ...) body ...+)
       #'((lambda (var-id ...) body ...) rhs-expr ...)]))

One minor difference is the use of ...+ in the pattern; ... means match zero or more repetitions of the preceding pattern; ...+ means match one or more. Only ... may be used in the template, however.

The first step toward validation and high-quality error reporting is annotating each of the macro’s pattern variables with the syntax class that describes its acceptable syntax. In mylet, each variable must be an identifier (id for short) and each right-hand side must be an expr (expression). An annotated pattern variable is written by concatenating the pattern variable name, a colon character, and the syntax class name.For an alternative to the “colon” syntax, see the ~var pattern form.

> (define-syntax (mylet stx)
    (syntax-parse stx
      [(_ ((var:id rhs:expr) ...) body ...+)
       #'((lambda (var ...) body ...) rhs ...)]))
Note that the syntax class annotations do not appear in the template (i.e., var, not var:id).

The syntax class annotations are checked when we use the macro.
> (mylet ([a 1] [b 2]) (+ a b))

3

> (mylet (["a" 1]) (add1 a))

mylet: expected identifier at: "a"

The expr syntax class does not actually check that the term it matches is a valid expression—that would require calling that macro expander. Instead, expr just means not a keyword.
> (mylet ([a #:whoops]) 1)

mylet: expected expression at: #:whoops

Also, syntax-parse knows how to report a few kinds of errors without any help:
> (mylet ([a 1 2]) (* a a))

mylet: unexpected term at: 2

There are other kinds of errors, however, that this macro does not handle gracefully:
> (mylet (a 1) (+ a 2))

mylet: bad syntax at: (mylet (a 1) (+ a 2))

It’s too much to ask for the macro to respond, “This expression is missing a pair of parentheses around (a 1).” The pattern matcher is not that smart. But it can pinpoint the source of the error: when it encountered a it was expecting what we might call a “binding pair,” but that term is not in its vocabulary yet.

To allow syntax-parse to synthesize better errors, we must attach descriptions to the patterns we recognize as discrete syntactic categories. One way of doing that is by defining new syntax classes:Another way is the ~describe pattern form.

> (define-syntax (mylet stx)
  
    (define-syntax-class binding
      #:description "binding pair"
      (pattern (var:id rhs:expr)))
  
    (syntax-parse stx
      [(_ (b:binding ...) body ...+)
       #'((lambda (b.var ...) body ...) b.rhs ...)]))

Note that we write b.var and b.rhs now. They are the nested attributes formed from the annotated pattern variable b and the attributes var and rhs of the syntax class binding.

Now the error messages can talk about “binding pairs.”
> (mylet (a 1) (+ a 2))

mylet: expected binding pair at: a

Errors are still reported in more specific terms when possible:
> (mylet (["a" 1]) (+ a 2))

mylet: expected identifier

  parsing context:

   while parsing binding pair at: "a"

There is one other constraint on the legal syntax of mylet. The variables bound by the different binding pairs must be distinct. Otherwise the macro creates an illegal lambda form:
> (mylet ([a 1] [a 2]) (+ a a))

lambda: duplicate argument name at: a

Constraints such as the distinctness requirement are expressed as side conditions, thus:
> (define-syntax (mylet stx)
  
    (define-syntax-class binding
      #:description "binding pair"
      (pattern (var:id rhs:expr)))
  
    (syntax-parse stx
      [(_ (b:binding ...) body ...+)
       #:fail-when (check-duplicate-identifier
                    (syntax->list #'(b.var ...)))
                   "duplicate variable name"
       #'((lambda (b.var ...) body ...) b.rhs ...)]))
> (mylet ([a 1] [a 2]) (+ a a))

mylet: duplicate variable name at: a

The #:fail-when keyword is followed by two expressions: the condition and the error message. When the condition evaluates to anything but #f, the pattern fails. Additionally, if the condition evaluates to a syntax object, that syntax object is used to pinpoint the cause of the failure.

Syntax classes can have side conditions, too. Here is the macro rewritten to include another syntax class representing a “sequence of distinct binding pairs.”
> (define-syntax (mylet stx)
  
    (define-syntax-class binding
      #:description "binding pair"
      (pattern (var:id rhs:expr)))
  
    (define-syntax-class distinct-bindings
      #:description "sequence of distinct binding pairs"
      (pattern (b:binding ...)
               #:fail-when (check-duplicate-identifier
                            (syntax->list #'(b.var ...)))
                           "duplicate variable name"
               #:with (var ...) #'(b.var ...)
               #:with (rhs ...) #'(b.rhs ...)))
  
    (syntax-parse stx
      [(_ bs:distinct-bindings . body)
       #'((lambda (bs.var ...) . body) bs.rhs ...)]))
Here we’ve introduced the #:with clause. A #:with clause matches a pattern with a computed term. Here we use it to bind var and rhs as attributes of distinct-bindings. By default, a syntax class only exports its patterns’ pattern variables as attributes, not their nested attributes.The alternative would be to explicitly declare the attributes of distinct-bindings to include the nested attributes b.var and b.rhs, using the #:attribute option. Then the macro would refer to bs.b.var and bs.b.rhs.

Alas, so far the macro only implements half of the functionality offered by Racket’s let. We must add the “named-let” form. That turns out to be as simple as adding a new clause:

> (define-syntax (mylet stx)
  
    (define-syntax-class binding
      #:description "binding pair"
      (pattern (var:id rhs:expr)))
  
    (define-syntax-class distinct-bindings
      #:description "sequence of distinct binding pairs"
      (pattern (b:binding ...)
               #:fail-when (check-duplicate-identifier
                            (syntax->list #'(b.var ...)))
                           "duplicate variable name"
               #:with (var ...) #'(b.var ...)
               #:with (rhs ...) #'(b.rhs ...)))
  
    (syntax-parse stx
      [(_ bs:distinct-bindings body ...+)
       #'((lambda (bs.var ...) body ...) bs.rhs ...)]
      [(_ loop:id bs:distinct-bindings body ...+)
       #'(letrec ([loop (lambda (bs.var ...) body ...)])
           (loop bs.rhs ...))]))
We are able to reuse the distinct-bindings syntax class, so the addition of the “named-let” syntax requires only three lines.

But does adding this new case affect syntax-parse’s ability to pinpoint and report errors?
> (mylet ([a 1] [b 2]) (+ a b))

3

> (mylet (["a" 1]) (add1 a))

mylet: expected identifier

  parsing context:

   while parsing binding pair

   while parsing sequence of distinct binding pairs at: "a"

> (mylet ([a #:whoops]) 1)

mylet: expected expression

  parsing context:

   while parsing binding pair

   while parsing sequence of distinct binding pairs at:

#:whoops

> (mylet ([a 1 2]) (* a a))

mylet: unexpected term

  parsing context:

   while parsing binding pair

   while parsing sequence of distinct binding pairs at: 2

> (mylet (a 1) (+ a 2))

mylet: expected binding pair

  parsing context:

   while parsing sequence of distinct binding pairs at: a

> (mylet ([a 1] [a 2]) (+ a a))

mylet: duplicate variable name

  parsing context:

   while parsing sequence of distinct binding pairs at: a

The error reporting for the original syntax seems intact. We should verify that the named-let syntax is working, that syntax-parse is not simply ignoring that clause.
> (mylet loop ([a 1] [b 2]) (+ a b))

3

> (mylet loop (["a" 1]) (add1 a))

mylet: expected identifier

  parsing context:

   while parsing binding pair

   while parsing sequence of distinct binding pairs at: "a"

> (mylet loop ([a #:whoops]) 1)

mylet: expected expression

  parsing context:

   while parsing binding pair

   while parsing sequence of distinct binding pairs at:

#:whoops

> (mylet loop ([a 1 2]) (* a a))

mylet: unexpected term

  parsing context:

   while parsing binding pair

   while parsing sequence of distinct binding pairs at: 2

> (mylet loop (a 1) (+ a 2))

mylet: expected binding pair

  parsing context:

   while parsing sequence of distinct binding pairs at: a

> (mylet loop ([a 1] [a 2]) (+ a a))

mylet: duplicate variable name

  parsing context:

   while parsing sequence of distinct binding pairs at: a

How does syntax-parse decide which clause the programmer was attempting, so it can use it as a basis for error reporting? After all, each of the bad uses of the named-let syntax are also bad uses of the normal syntax, and vice versa. And yet the macro doen not produce errors like “mylet: expected sequence of distinct binding pairs at: loop.”

The answer is that syntax-parse records a list of all the potential errors (including ones like loop not matching distinct-binding) along with the progress made before each error. Only the error with the most progress is reported.

For example, in this bad use of the macro,
> (mylet loop (["a" 1]) (add1 a))

mylet: expected identifier

  parsing context:

   while parsing binding pair

   while parsing sequence of distinct binding pairs at: "a"

there are two potential errors: expected distinct-bindings at loop and expected identifier at "a". The second error occurs further in the term than the first, so it is reported.

For another example, consider this term:
> (mylet (["a" 1]) (add1 a))

mylet: expected identifier

  parsing context:

   while parsing binding pair

   while parsing sequence of distinct binding pairs at: "a"

Again, there are two potential errors: expected identifier at (["a" 1]) and expected identifier at "a". They both occur at the second term (or first argument, if you prefer), but the second error occurs deeper in the term. Progress is based on a left-to-right traversal of the syntax.

A final example: consider the following:
> (mylet ([a 1] [a 2]) (+ a a))

mylet: duplicate variable name

  parsing context:

   while parsing sequence of distinct binding pairs at: a

There are two errors again: duplicate variable name at ([a 1] [a 2]) and expected identifier at ([a 1] [a 2]). Note that as far as syntax-parse is concerned, the progress associated with the duplicate error message is the second term (first argument), not the second occurrence of a. That’s because the check is associated with the entire distinct-bindings pattern. It would seem that both errors have the same progress, and yet only the first one is reported. The difference between the two is that the first error is from a post-traversal check, whereas the second is from a normal (i.e., pre-traversal) check. A post-traveral check is considered to have made more progress than a pre-traversal check of the same term; indeed, it also has greater progress than any failure within the term.

It is, however, possible for multiple potential errors to occur with the same progress. Here’s one example:
> (mylet "not-even-close")

mylet: expected identifier or expected sequence of distinct

binding pairs at: "not-even-close"

In this case syntax-parse reports both errors.

Even with all of the annotations we have added to our macro, there are still some misuses that defy syntax-parse’s error reporting capabilities, such as this example:
> (mylet)

mylet: expected more terms at: (mylet)

The philosophy behind syntax-parse is that in these situations, a generic error such as “bad syntax” is justified. The use of mylet here is so far off that the only informative error message would include a complete recapitulation of the syntax of mylet. That is not the role of error messages, however; it is the role of documentation.

This section has provided an introduction to syntax classes, side conditions, and progress-ordered error reporting. But syntax-parse has many more features. Continue to the Examples section for samples of other features in working code, or skip to the subsequent sections for the complete reference documentation.

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