CODEWITHSUNDEEP
schemeundefined
zvavybir
zvavybir

Reputation: 1058

How many undefined values do Scheme have?

I read on https://ds26gte.github.io/tyscheme/index-Z-H-11.html tree.girth returns "an undefined value". Is this right? Have Scheme really multiply undefined values?

Upvotes: 1

Views: 214

Answers (2)

alinsoar
alinsoar

Reputation: 15803

Undefined means that you must not use the return value -- it can be anything.

In scheme a command like display will return the #!unspecific symbol and in racket(i.e. the old plt-scheme) the void symbol. You should not use the value #!unspecific returned by display. This means undefined value.

% mit-scheme
MIT/GNU Scheme running under GNU/Linux
Type `^C' (control-C) followed by `H' to obtain information about interrupts.

Copyright (C) 2019 Massachusetts Institute of Technology

1 ]=> (display (display "ok"))
ok#!unspecific
;Unspecified return value

% racket
Welcome to Racket v7.3.
>  (display (display "ok"))
ok#<void>
>

Upvotes: 1

Barmar
Barmar

Reputation: 782130

"Undefined" is not a specific value. What it's saying is that the value returned is not defined by the language specification, so it could be anything.

Some implementations may have a specific object that they return in these situations, to aid with debugging. But there's no requirement to do so. And whether different instances of these object are equal to each other is not specified -- it's not like #false, which is always the same object.

Upvotes: 3

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