There is no immutable support in Squeak.
Smalltalk custom is to avoid overwriting literals like String Array...
...Same for instances of other classes like Point Rectangle which are
likely to be shared by several objects.
Most algorithms will rather modify and answer a copy.
This default behaviour has a cost (creates lots of objects) but is
fool proof (no danger to overwrite a shared information).

For DES, I don't know, maybe it's just an optimization you must be aware of.
In that case, you are responsible for creating the copy by yourself
when required.

Nicolas

2010/7/25 Rob Withers <reefedjib at yahoo.com>:
> Thanks, that makes sense.  Isn't that a bug since Strings are mutable?
>
> --------------------------------------------------
> From: "Nicolas Cellier" <nicolas.cellier.aka.nice at gmail.com>
> Sent: Sunday, July 25, 2010 3:51 AM
> To: "The general-purpose Squeak developers list"
> <squeak-dev at lists.squeakfoundation.org>
> Subject: Re: [squeak-dev] TestRunner not assigning strings/remembering old
> values of temp vars.
>
>> Of course, 'squeaker' is a literal stored in the CompiledMethod.
>> If you write over it, then you break the literal for next method
>> execution.
>>
>> Each time you evaluate in a workspace, you create a new CompiledMethod
>> and a new literal.
>> When you compile in a class, that's different because you compiled
>> only once and execute many.
>>
>> Nicolas
>>
>> 2010/7/25 Rob Withers <reefedjib at yahoo.com>:
>>>
>>> I am testing the CryptoDESTest.  I am getting some strange behavior. When
>>> I
>>> run the following code in a Workspace, repeatably, I get the same
>>> results:
>>>
>>> code:
>>>
>>>       | plain key d |
>>>       plain := 'squeaker'.
>>>       key := 'hacking!'.
>>>       Transcript cr; show: 'plain: ', plain printString.
>>>       d := DES key: key.
>>>       d encryptBlock: plain.
>>>       Transcript cr; show: 'plain: ', plain printString.
>>>       self assert: plain asByteArray = #[64 222 70 13 59 145 76 186].
>>>       plain destroy.
>>>
>>> output:
>>>
>>> plain: 'squeaker'
>>> plain: '@ÞF
>>> ;‘Lº'
>>> plain: 'squeaker'
>>> plain: '@ÞF
>>> ;‘Lº'
>>> plain: 'squeaker'
>>> plain: '@ÞF
>>> ;‘Lº'
>>>
>>> However, when I run this as a test method:
>>>
>>> testDES3
>>>       | plain key d |
>>>       plain := 'squeaker'.
>>>       key := 'hacking!'.
>>>       Transcript cr; show: 'plain: ', plain printString.
>>>       d := DES key: key.
>>>       d encryptBlock: plain.
>>>       Transcript cr; show: 'plain: ', plain printString.
>>>       self assert: plain asByteArray = #[64 222 70 13 59 145 76 186].
>>>              plain destroy.
>>>
>>> I get the following output and the test fails:
>>>
>>> plain: 'squeaker'
>>> plain: '@ÞF
>>> ;‘Lº'
>>> plain: '        '
>>> plain: '£ƒ¼ïùmÞ$'
>>> plain: '£ƒ¼ïùmÞ$'
>>> plain: '§,}ºé¨öú'
>>> plain: '§,}ºé¨öú'
>>> plain: '3qX¦wRË|'
>>> plain: '3qX¦wRË|'
>>> plain: 'Ê0ê^ó{ì '
>>> plain: 'Ê0ê^ó{ì '
>>> plain: 'çî³›,ίw'
>>>
>>> Why is test runner remembering the old value and not setting it with:
>>> plain
>>> := 'squeaker'.?
>>>
>>> TIA,
>>> Rob
>>>
>>>
>>
>
>