How to redefine Scheme/Racket fundamental forms?

Question

Racket has built-in fundamental form 2-arm if, but it doesn't have the word else, so I want to add the else word to it.

This code works:

(require syntax/parse/define)
(define-syntax-rule (myif Cond Form1 else Form2)
  (if Cond Form1 Form2)
)

(myif #t (displayln 1) else (displayln 2))

However myif is undesired as keyword, changing it to if raises error:

if: use does not match pattern: (if Cond Form1 else Form2)
in: (if #t (displayln 1) (displayln 2))

How to redefine the form if?

When you redefine `if`, make sure you change uses of the "old" `if` so that they don't accidentally point to the wrong one. In the new definition you want to delegate to the old `if`, so I recommend importing the old if with `(require (only-in racket/base [if old-if]))`, then using `old-if` where you intend — Alex Knauth, Feb 12 '20 at 04:31

score 2 · Accepted Answer · answered Feb 12 '20 at 17:34

The answer above is "correct" at some level, but there are some mistakes.

define-syntax-rule is not from syntax/parse/define. If you want to use define-syntax-rule, there's no need to (require syntax/parse/define).
What will happen when I call (if #t 1 2 3) with your if? else in your macro is a pattern variable, so it can match against anything. Do you intend to allow this to happen? If not, this is where you can use the features of syntax/parse/define. You can write:
```
(require syntax/parse/define)
(define-simple-macro (if test-expr:expr 
                         {~datum then} 
                         then-expr:expr 
                         {~datum else} 
                         else-expr:expr)
  (racket:if test-expr then-expr else-expr))
```
So (if 1 then 2 else 3) will expand to (racket:if 1 2 3), but (if 1 0 2 0 3) will fail with an error.

score 1 · Answer 2 · answered Feb 12 '20 at 04:35

Thanks @AlexKnauth for the comment, I can redefine if as below:

(require syntax/parse/define)
(require (only-in racket/base [if r-if]))

(define-syntax-rule (if Cond Form1 else Form2)
  (r-if Cond Form1 Form2)
)

(if #t (displayln 1) else (displayln 2))

How to redefine Scheme/Racket fundamental forms?

2 Answers2