Stack overflow today led me to something … very strange. I feel like I must be
missing something crushingly obvious.
To reproduce: start racket at the command line or hit “run” on a definitions
window containing only “#lang racket”.
Then, paste:
(define map (lambda (l f)
(cond
((null? l) '())
(else (cons (f (car l)) (map (cdr l) f)))
)
))
(Note that this version of map takes the function last.) Then, paste:
(map '(3 4 5) add1)
See this error:
map: contract violation
expected: procedure?
given: '(4 5)
argument position: 1st
other arguments…:
Note that the error occurs on the recursive call. This means that despite being
inside of a binding of the name map, the function is using the built-in
function.
When I try doing the same thing with a value defined in the empty program (with
the name’f’), I see the error
define-values: assignment disallowed;
cannot re-define a constant
constant: f
Which leads me to the suspicion that maybe possibly this message is supposed to
be shown for the re-definition of ‘map’, as well?
Very confused,
John
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