On Sun, Jan 13, 2019 at 12:37 AM Hassan Shahin <hsha...@gmail.com> wrote:
> Thanks Jack and Mike!
>
> You are right. Arguments to procedures will be evaluated before the
> invocation of the procedure.
>
This is true, but it's not really the issue in your case. Even in #lang
lazy, which does not eagerly evaluate procedure arguments, your program
would still be an error, because the expression `John` has no meaning at
all unless you define it. Try the following in both #lang racket and #lang
lazy:
--- definitions window ---
#lang lazy ;; or racket
(define type-of (lambda (item other-message)
(cond
[(pair? item) 'pair]
[(null? item) 'empty-list]
[(number? item) 'number]
[(symbol? item) 'symbol]
[else
other-message
'some-other-type])))
--- interactions window ---
(type-of #\e (displayln "hello"))
(type-of (cons 2 3) (displayln "hello"))
(type-of John (displayln "hello"))
---
The `John` example will fail in both languages, but you'll observe the
difference with the other two examples.
- Jon
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