[racket] lambda as default value to dict-ref

If the failure-result is a procedure, it is called without arguments. I
quote part of the doc:

quote
•If failure-result is a procedure, it is called (through a tail call) with
no arguments to produce the result. 
endquote

Jos
-----Original Message-----
From: users-bounces at racket-lang.org [mailto:users-bounces at racket-lang.org]
On Behalf Of J G Cho
Sent: sábado, 16 de febrero de 2013 3:55
To: Racket mailing list
Subject: [racket] lambda as default value to dict-ref

So, I have a dict A that looks like (list ("string as key" . (lambda
(arg) ...) ...)

I do ((dict-ref A key (lambda (_) "wrong key")) some-params).

When keys are valid the above work as expected. When the key does not
find any, I was expecting

((lambda (_) "wrong key") some-params) but it fails with this msg:

 arity mismatch;
 the expected number of arguments does not match the given number
  expected: 1
  given: 0

Upon further inspection,

(dict-ref A key (lambda (_) "wrong key"))  does not seem to return the
lambda but seems to invoke it. Why so? What am I failing to understand
here?


Confused,

jGc
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