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3.4 Expression Wrapper: #%expression

Produces the same result as

. Using

forces the parsing of a form as an expression.

Examples:

The

form is helpful in recursive definition contexts where expanding a subsequent definition can provide compile-time information for the current expression. For example, consider a

macro that simply records some symbols at compile-time in a given identifier.

and then a variant of

that checks to make sure the symbols used in the expression match those given in the earlier definition:

If the definition follows the use like this, then the

macro does not have a chance to bind

and the

macro signals an error:

> (let ()     (sym-case land-creatures 'bear               [(bear) 1]               [(fox) 2])     (define-sym-case land-creatures bear fox))

eval:11:0: sym-case: expected an identifier bound via

define-sym-case

  at: land-creatures

  in: (sym-case land-creatures (quote bear) ((bear) 1)

((fox) 2))

But if the

is wrapped in an

, then the expander does not need to expand it to know it is an expression and it moves on to the

expression.

> (let ()     (#%expression (sym-case sea-creatures 'whale                             [(whale) 1]                             [(squid) 2]))     (define-sym-case sea-creatures whale squid)     'more...)

'more...

Of course, a macro like

should not require its clients to add

; instead it should check the basic shape of its arguments and then expand to

wrapped around a helper macro that calls

and finishes the expansion.

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