Generators and Iterators a la Python
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Generators and Iterators a la Python
Anyone thinking about Python-like generators and iterators; generators can be thought of as "lazy" lists of mappings (simple or complex) of possibly infinite length.
The key innovation needed would be for APL built-in functions and operators, when faced with a user-defined class object, to expect and call certain class members, e.g. myclass.next, as well as to respond to certain signals (modeling Python StopIteration). While there are many elegant uses of generators and iterators we just don't need in APL, there are some uses already in place in an ad hoc fashion -- note how selection works today with certain default class methods (where it lazily waits before calling the method until it knows which object components it needs).
The <yield> keyword in Python (which allows generators to be programmed as if regular (co-)functions), yielding a value, while maintaining the current state, rather then returning a value. It can be simulated using threads, but requires different user functions to stay out of each other's way (e.g. in token management or using local variables to synchronize).
See:http://www.python.org/dev/peps/pep-0255/ (also, less elegantly, in many other languages).
Seems easy enough! Devils in the details.
The key innovation needed would be for APL built-in functions and operators, when faced with a user-defined class object, to expect and call certain class members, e.g. myclass.next, as well as to respond to certain signals (modeling Python StopIteration). While there are many elegant uses of generators and iterators we just don't need in APL, there are some uses already in place in an ad hoc fashion -- note how selection works today with certain default class methods (where it lazily waits before calling the method until it knows which object components it needs).
The <yield> keyword in Python (which allows generators to be programmed as if regular (co-)functions), yielding a value, while maintaining the current state, rather then returning a value. It can be simulated using threads, but requires different user functions to stay out of each other's way (e.g. in token management or using local variables to synchronize).
See:http://www.python.org/dev/peps/pep-0255/ (also, less elegantly, in many other languages).
Seems easy enough! Devils in the details.
- petermsiegel
- Posts: 160
- Joined: Thu Nov 11, 2010 11:04 pm
Re: Generators and Iterators a la Python
While nowhere near as neat as primitive language constructs, some of this functionality may already be modelled using namespaces, bound as operands or left arguments, to maintain persistent local state. See:
Function memoization: http://dfns.dyalog.com/n_memo.htm
Editor undo/redo stacks: http://dfns.dyalog.com/n_UndoRedo.htm
and this experimental version of dyalog, which implemented closures:
Closures: http://dfns.dyalog.com/downloads/fre.pdf
Function memoization: http://dfns.dyalog.com/n_memo.htm
Editor undo/redo stacks: http://dfns.dyalog.com/n_UndoRedo.htm
and this experimental version of dyalog, which implemented closures:
Closures: http://dfns.dyalog.com/downloads/fre.pdf
- JohnS|Dyalog
Re: Generators and Iterators a la Python
I had seen the closures article, which was spot on; closures would be just the tool needed. But they aren't part of the product, right?
- petermsiegel
- Posts: 160
- Joined: Thu Nov 11, 2010 11:04 pm
Re: Generators and Iterators a la Python
Closures are not part of the product. You can model persistent local state by binding a namespace as left argument/operand:
See http://dfns.dyalog.com/n_memo.htm for more examples.
John.
- Code: Select all
next←(⎕ns'')∘{ ⍝ space bound as left arg.
0=⍵:⍺.n←0 ⍝ next 0: reset number stream.
(⍳⍺.n+←⍵)+⍺.n ⍝ next ⍵: return next ⍵ numbers.
}
next 0 ⍝ reset count.
next 1 ⍝ next number.
1
next 1 ⍝ next number.
2
next 2 ⍝ next two numbers.
3 4
next 1 ⍝ next number.
5
next 0 ⍝ reset count.
next 3 ⍝ next three numbers,
1 2 3
See http://dfns.dyalog.com/n_memo.htm for more examples.
John.
- JohnS|Dyalog
Re: Generators and Iterators a la Python
That's a neat technique.
Is there a reason I can't fix the 'next' function in Dyalog 13.1 for Linux?
Is there a reason I can't fix the 'next' function in Dyalog 13.1 for Linux?
- David Lamkins
- Posts: 21
- Joined: Sun Aug 26, 2012 7:08 am
Re: Generators and Iterators a la Python
BTW, the technique works for me if I break it down into two functions, like this:
It seems that my Linux version of Dyalog can't parse composition as part of a d-function definition.
- Code: Select all
next∆←{
0=⍵:⍺.n←0
(⍳⍺.n+←⍵)+⍺.n
}
next←(⎕ns'')∘next∆
It seems that my Linux version of Dyalog can't parse composition as part of a d-function definition.
- David Lamkins
- Posts: 21
- Joined: Sun Aug 26, 2012 7:08 am
Re: Generators and Iterators a la Python
Hi David,
Our session doesn't allow you to join together multiple lines into one action. But, you can get this derived function next if you use diamonds and type it on one line in the session like this:
Regards,
Vince
Our session doesn't allow you to join together multiple lines into one action. But, you can get this derived function next if you use diamonds and type it on one line in the session like this:
- Code: Select all
next←(⎕ns'')∘{ 0=⍵:⍺.n←0 ⋄ (⍳⍺.n+←⍵)+⍺.n}
Regards,
Vince
- Vince|Dyalog
- Posts: 443
- Joined: Wed Oct 01, 2008 9:39 am
Re: Generators and Iterators a la Python
Apologies, I forgot that I was using a short-hand. Vince's one-liner-with-diamonds is good for short functions and your 2-stage definition is better for larger ones.
Note that you can also define next as an operator, which leaves the left argument of the resulting derived function free.
NB: In the above, the multi-line D-fn may not be entered directly into the session.
Use )ed next∆ to define it.
Note that you can also define next as an operator, which leaves the left argument of the resulting derived function free.
- Code: Select all
next∆←{⍺←0 ⍝ default: no reset:
⍺:⍺⍺.n←⍵-⍳1 ⍝ reset sequence to ⍵.
(⍳⍺⍺.n+←⍵)+⍺⍺.n ⍝ next ⍵ numbers.
}
next ← (⎕ns'')next∆ ⍝ NB: no '∘'
1 next 0 ⍝ reset sequence to 0.
next 2 ⍝ next 2 numbers
0 1
next 0 ⍝ next 0 numbers
next 3 ⍝ next 3 numbers
2 3 4
1 next 42 ⍝ reset sequence to 42
next 2 ⍝ next 2 numbers
42 43
NB: In the above, the multi-line D-fn may not be entered directly into the session.
Use )ed next∆ to define it.
- JohnS|Dyalog
Re: Generators and Iterators a la Python
Thank you, John and Vince.
Your replies had me scratching my head for a moment. I know that dfns must be defined on one line in the session. I tried that this morning; of course it works.
Yesterday, though, I was attempting to define 'next' in an editor. This (your original version using the namespace composed with the function) gives me a "Can't Fix" message in the mode line regardless of whether the definition is on one line or not.
It seems, therefore, that the editor and the session have different syntax requirements for dfns. Is this expected?
Your replies had me scratching my head for a moment. I know that dfns must be defined on one line in the session. I tried that this morning; of course it works.
Yesterday, though, I was attempting to define 'next' in an editor. This (your original version using the namespace composed with the function) gives me a "Can't Fix" message in the mode line regardless of whether the definition is on one line or not.
It seems, therefore, that the editor and the session have different syntax requirements for dfns. Is this expected?
- David Lamkins
- Posts: 21
- Joined: Sun Aug 26, 2012 7:08 am
Re: Generators and Iterators a la Python
It seems, therefore, that the editor and the session have different syntax requirements for dfns. Is this expected?
Yes, in this case, the session is being used to name the result of the evaluation of an expression:
half ← ÷2 ⍝ naming result of function applied to array argument
sum ← +/ ⍝ naming result of operator applied to function operand
next ← (⎕ns'')∘{...} ⍝ naming result of operator applied to operands
The editor can define only the components (arrays, functions, operators) of such expressions.
John
PS. great advances have often arisen from questions such as these.
- JohnS|Dyalog
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