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Re: Set::setps error? Twitter.m's OOP-like approach?

• To: mathgroup at smc.vnet.net
• Subject: [mg104575] Re: Set::setps error? Twitter.m's OOP-like approach?
• From: dh <dh at metrohm.com>
• Date: Wed, 4 Nov 2009 01:35:34 -0500 (EST)
• References: <hconr9$1lj$1@smc.vnet.net>

Hi Erik,

what you want to do is a bit tricky.

By: deposit[a, 25] you want to assigne to the variable "a" an expression 

like: Account[..,..]

By default all arguments to a function are evaluated prior to calling 

the function. Therefore your variable "account" does not contain "a" but 

the expression "Account["test", 100]". To obtain the variable name you 

must prevent the evaluation by e.g. specifying; 

SetAttributes[deposit,HoldFirst].

Inside the function you must again take precautiion against evaluation 

of the variable by specifying e.g.:

Unevaluated[account] =

As you have not defined an addition rule for "Account" we need to create 

the full expression on the right side of the assignment:

Unevaluated[account] = Account[account[[1]], account[[2]] + amount]

Here "account" contains the evaluated "a", namely: "Account["test", 100]"

Therefore, deposit would be declared e.g. like:



SetAttributes[deposit, HoldFirst];

deposit[account_, amount_] :=

  Unevaluated[account] = Account[account[[1]], account[[2]] + amount]



Daniel



Erik Max Francis wrote:

> I was experimenting with vaguely OOP-like wrappers around data similar 

> to how Twitter.m does things:

> 

> 	http://blog.wolfram.com/2009/04/30/twittering-with-mathematica/

> 

> I was experimenting with "mutable objects" and I'm running into a 

> scoping problem of some type that I don't understand.  Here's some 

> sample code creating a wrapper around an "account balance":

> 

> In[1]:= {NAME, BALANCE} = Range[2]

> 

> Out[1]= {1, 2}

> 

> In[2]:= create[name_, balance_] := Account[name, balance]

> 

> In[3]:= getName[account_Account] := account[[NAME]]

> 

> In[4]:= getBalance[account_Account] := account[[BALANCE]]

> 

> In[5]:= deposit[account_Account, amount_] :=

>   account[[BALANCE]] += amount

> 

> The deposit function doesn't work, though:

> 

> In[6]:= a = create["test", 100]

> 

> Out[6]= Account["test", 100]

> 

> In[7]:= getBalance[a]

> 

> Out[7]= 100

> 

> In[8]:= deposit[a, 25]

> 

> During evaluation of In[8]:= Set::setps: Account[test,100] in the part 

> assignment is not a symbol. >>

> 

> Out[8]= 125

> 

> In[9]:= getBalance[a]

> 

> Out[9]= 100

> 

> But here's what I'm confused; if I do what getBalance does manually, it 

> works fine:

> 

> In[10]:= a[[BALANCE]] += 25

> 

> Out[10]= 125

> 

> In[11]:= a

> 

> Out[11]= Account["test", 125]

> 

> The Set::setps error is about Blocks so I suppose I'm running into some 

> scoping issue but I don't follow what it is.  What am I missing here? 

> What's the right way to write getBalance?

> 

> Do Mathematica users find Twitter.m's approach to wrapping around 

> objects palatable in the first place?  (My eventual application is for a 

> wrapper around tensor objects where multiple things have to be carted 

> around and the normal display of such objects would be far too ungainly 

> to be acceptable anyway.)

> 

> Thanks.

>