CODEWITHSUNDEEP
macrosschemeracket
rnso
rnso

Reputation: 24555

Macro to write -n as negative of n in Racket

If n is defined as 5, (- n) will give a value of -5. Can there be a macro to identify "-n" as negative of n or (- n) in Racket language? 

> (define n 5)
> n
5
> (- n)
-5
> (-n)
. . -n: undefined;
 cannot reference an identifier before its definition
> -n
. . -n: undefined;
 cannot reference an identifier before its definition
>

Upvotes: 0

Views: 920

Answers (1)

soegaard
soegaard

Reputation: 31147

Yes. If we assume you have no variables in your program with an identifier that begins with a dash you can abuse #%top.

The expression -n is a variable reference to an unbound variable. The expander will turn -n into (#%top . -n).

If you in module negative.rkt write a macro named my-top and provide it using (provide (rename-out [my-top #%top])) then you can write (require "negative.rkt") to use your own version of #%top.

The definition of my-top is something like:

if the input identifier x begins with - then turn -something into (- something) otherwise return (#%top . x).

See this question for an example of how to redefine #%top: Macro of [S:N] for in-range in Racket

Upvotes: 2

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