[Jprogramming] A neat OOP trick with adverbs

['Pascal Jasmin' via Programming]

Placing this in a file and then loading

coclass 'myclass'
a =: 2 NB. member variable inside class or object.  Noun intended.
A =: 1 : ' a=: u a' NB. adverb that will modify member variable

in console (base locale) you can do:

 a =. conew 'myclass' NB. a is variable in base locale
 +: A__a 
4 
 3&+ A__a 
7 
 a__a7
 +: A_myclass_ NB. uses class variable instead of instance variable. future conew's would use 4 as the initial instance val
4

 In terms of what you were trying to do, =. is always local to the function =: keeps variables within a locale.  I'm not completely sure why top level (console) allows =., but I suspect it is just an added debugging convenience.

The intended benefit of the adverb pattern is allowing your code to receive variable verbs/functions to apply to the current state of your object, and likely to modify its state. Amending an internal table for example.

But another example would be moving the x/y coordinates of a game/visual object

 2 3"_ A__a 
2 3


 2 3&+ A__a [ 2 3"_ A__a 
4 6 


I'm passing a move order that is relative to the existing state.  While there is always the alternative of this longer expression:

a__a =: 2 3 + a__a

the big advantage of the adverb approach is that you can extend the adverb to not just make the move, but also check for collisions, send a rendering command, or perhaps generate a more complex timelapse of the movement from the old and destination states, and those features can be added in 2.0/3.0 iterations without affecting all of the "client"/consumer code that just cares about moving tiles around. 



----- Original Message -----
From: Jon Hough <jghough at outlook.com>
To: "programming at jsoftware.com" <programming at jsoftware.com>
Cc: 
Sent: Tuesday, December 9, 2014 11:15 AM
Subject: Re: [Jprogramming] A neat OOP trick with adverbs




I was playing with your adverbs.Any idea why the following doesn't work?
coclass 'myclass'empty =: ''classname =: myclass
A_myclass_=:    1 :'a =. empty&u classname' 



   conew A


error is:


I get the following error:
|value error: create__w


|       create__w x


My intention was to instantiate 'a' as an instance of myclass (but also keeping it private for whatever reason). 









> Date: Tue, 9 Dec 2014 03:31:23 +0000
> From: programming at jsoftware.com
> To: programming at jsoftware.com
> Subject: [Jprogramming] A neat OOP trick with adverbs
> 
> adverbs can return nouns, though most are designed to return a verb.
> 
> 1 : 'm + y'  NB. a verb because it accesses y
> 1 : 'u 2'  NB. returns a noun because it doesn't access y even though it uses a verb as adverb parameter
> 
> Consider this noun producing adverb,
> A_z_ =: 1 : 'a =: u a'
> 
> its defined in z, just for convenience.  It applies its verb parameter to a and stores the result in a.  The value of a it will use is whatever locale it is qualified with.
> 
>   a=.1 
>   a_b_ =: 3 
> 
>   +: A 
> 2 
> 
>  +: A_b_ 
> 6 
>  a_b_ 
> 6 
> 
>  a&+ A_b_ NB. the a parameter is taken from caller's locale as u is parsed before it is passed to A_b_
> 8 
> 
> 
>  a_b_ =: i.5 
>  a&(0}) A_b_ NB. sending parameters to amend in place
> 2 1 2 3 4 
> 
> 
> OOP approach often involves creating side effects in class instances.  Noun returning adverbs can access all instance variables (or other program data), and make any side effects it wants.  For instance, it can save or backup data to disk in the last amend example.
> 
> The neat part is that with class defined adverbs, you can send verb messages to your objects.  Even though they are verbs, they can be bound with many caller supplied nouns (such as the 2 parameters to }), and in a tacit expression caller variables get fixed into the supplied verb.  Its also neat, because many other uses of adverbs in objects are unintuitively problematic in their parsing.
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