Discussion:
Randoms as LazyLists
(too old to reply)
'Rupert Smith' via Elm Discuss
2017-11-10 16:45:53 UTC
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I have a function that takes a random Generator and turns it into a
LazyList. The reason for this is so that I can combine random streams
together with other lazy streams that I have using the Lazy.List library.

The function uses the exposed Lazy.List.LazyListView type, which I would
rather avoid if it makes sense to:

generate : Generator a -> Seed -> LazyList a
generate generator seed =
let
( value, newSeed ) =
Random.step generator seed
in
lazy <|
\() ->
Cons value (generate generator newSeed)

This version uses iterate on a (a, Seed) -> (a, Seed) function, but then
requires a map on the tuple to extract just the random items:

generate2 : Generator a -> Seed -> LazyList a
generate2 generator seed =
let
firstValueSeed =
Random.step generator seed

step ( value, seed ) =
Random.step generator seed
in
LL.iterate step firstValueSeed
|> LL.map first

Is there some obvious way of doing this without the extra map operation? or
perhaps that will compile down efficiently anyway.

Does there perhaps need to be a variation on the Lazy.List.cons constructor
that takes a '() -> LazyList a' argument to allow the cons operation to be
passed as a continuation?

Rupert
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'Rupert Smith' via Elm Discuss
2017-11-10 16:58:32 UTC
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Post by 'Rupert Smith' via Elm Discuss
The function uses the exposed Lazy.List.LazyListView type, which I would
The documentation says:

"It is not recommended to work with this type directly. Try working solely
with the provided functions in the package."

But I wonder, does this really make sense? Its not like this library is
going to swap out that representation for another one at a later time is
it? In which case exposing the type and working with it directly, which is
needed for pattern matching anyway, is no different than working with say
the List type directly.
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art yerkes
2017-11-11 18:33:34 UTC
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I always interpreted the warning in the documentation as "Don't use the
name LazyListView or the Nil and Cons constructors, instead using head,
tail and headAndTail for most pattern matchng". I think it makes sense not
to use the internal representation given that Dict and Array change when
better implementations arise. Does any code elm code actually pattern
match these instead of using the functions? Always thought it was weird to
have this type exposed at all.
Post by 'Rupert Smith' via Elm Discuss
Post by 'Rupert Smith' via Elm Discuss
The function uses the exposed Lazy.List.LazyListView type, which I would
"It is not recommended to work with this type directly. Try working solely
with the provided functions in the package."
But I wonder, does this really make sense? Its not like this library is
going to swap out that representation for another one at a later time is
it? In which case exposing the type and working with it directly, which is
needed for pattern matching anyway, is no different than working with say
the List type directly.
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'Rupert Smith' via Elm Discuss
2017-11-13 09:48:41 UTC
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Post by art yerkes
I always interpreted the warning in the documentation as "Don't use the
name LazyListView or the Nil and Cons constructors, instead using head,
tail and headAndTail for most pattern matchng". I think it makes sense not
to use the internal representation given that Dict and Array change when
better implementations arise. Does any code elm code actually pattern
match these instead of using the functions? Always thought it was weird to
have this type exposed at all.
I cannot use the 'cons' constructor though, as it is not lazy enough:

generate : Generator a -> Seed -> LazyList a
generate generator seed =
let
( value, newSeed ) =
Random.step generator seed
in
cons value (generate generator newSeed)


Runs out of stack space, as the recursive step is evaluated eagerly.

I think it is strange to expose a type but recommend not using it. Either a
type is opaque and cannot be used directly, or exposed and it is perfectly
ok to use it.
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'Rupert Smith' via Elm Discuss
2017-11-14 10:15:19 UTC
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'iterate' is the right way to do what you're looking for. No idea why
they chose to expose that.
generate : Generator a -> Seed -> LazyList a
generate generator seed =
let start = Random.step generator seed in
LL.iterate
(\(v,s) -> Random.step generator s)
start
|> LL.map Tuple.first
Yes, this is the same code as I posted originally (generate2). It just
seemed overkill to have to apply a 'map first' when it can be accomplished
in a single pass using the Cons constructor.
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'Rupert Smith' via Elm Discuss
2017-11-14 10:20:26 UTC
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The LazyList library as it exists is seriously flawed as it crashes for
many cases for infinite length lazy lists, which is probably why there is
that caution. Please see my extensive PR which fixes almost every function
so they can handle infinite length lazy lists within the guidelines.
That is not so good, as infinite lists is much of the point of lazy lists.
Lets ping the package maintainer and find out why your PR is just sitting
there since February.
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W. Gordon Goodsman
2017-11-17 13:24:55 UTC
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Looks like this version of lazy-list and even lazy itself have been
depreciated, thus my PR is closed.

I also looks like the recommended non-memoizing version of lazy-list has
the same memory leaks for very long (infinite) lists, and thus needs my PR
applied to it.

The rational to depreciating these seems to be performance when applied to
their limited use in tests and I see that, as I tried to make changes to
Lazy that would improve its lack of performance when run on some JavaScript
engines that handle internal functions calling functions very poorly.

However, I strongly disagree with removing these memoization versions of
the libraries from the language, as just because they aren't needed for
tests doesn't mean they are never needed. Noah has suggested that one can
implement memoization themselves when needed, but hasn't explained how one
can do that and remain referentially transparent and not use JavaScript
without using Lazy, and if one does implement Lazy themselves, then the
resulting code won't be accepted for libraries because it contains
JavaScript.

This thread is probably not the place to discuss this, although it is going
to affect the objective of the OP, but I don't know where it should be
discussed since Noah has requested we don't discuss it on elm-dev.

I'll cease and desist about this if someone can show me how to write
referentially transparent deferred memoization of the result of a function
call without using Lazy or JavaScript.

On Nov 14, 2017 17:20, "'Rupert Smith' via Elm Discuss" <
Post by 'Rupert Smith' via Elm Discuss
The LazyList library as it exists is seriously flawed as it crashes for
many cases for infinite length lazy lists, which is probably why there is
that caution. Please see my extensive PR which fixes almost every function
so they can handle infinite length lazy lists within the guidelines.
That is not so good, as infinite lists is much of the point of lazy lists.
Lets ping the package maintainer and find out why your PR is just sitting
there since February.
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'Rupert Smith' via Elm Discuss
2017-11-20 10:35:44 UTC
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Post by W. Gordon Goodsman
The rational to depreciating these seems to be performance when applied to
their limited use in tests and I see that, as I tried to make changes to
Lazy that would improve its lack of performance when run on some JavaScript
engines that handle internal functions calling functions very poorly.
Gordon, it took me a bit of digging through various docs and updates on
elm-dev to understand it, but... the rational for deprecating elm-core/lazy
with memoization, is that it allowed recursive structures to be created on
the heap, and all other ways in which that was possible have been
eliminated. This is all described here:

https://gist.github.com/evancz/07436448b7d6c947f21742dab46d1218

It seems to be partly motivated by making garbage collection simpler -
presumably with WebAssembly in mind. I honestly don't know much of a real
advantage that is, but it seems that was the trade-off decision that has
already been made.
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W. Gordon Goodsman
2017-11-20 10:51:47 UTC
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Oh, I understand the rationale, but it limits the general usefulness of
lazy lists, as certain types of problems need memoization, and without Lazy
as it currently exists there is no way to accomplish it without JavaScript.

Just because the creators of the language don't see a need for it doesn't
mean it doesn't exist. Even for a lazy list of random numbers, no
memoization means that new random numbers will be computed every time the
list is scanned, meaning the lazy list won't appear to be static.

On Nov 20, 2017 17:35, "'Rupert Smith' via Elm Discuss" <
Post by 'Rupert Smith' via Elm Discuss
Post by W. Gordon Goodsman
The rational to depreciating these seems to be performance when applied
to their limited use in tests and I see that, as I tried to make changes to
Lazy that would improve its lack of performance when run on some JavaScript
engines that handle internal functions calling functions very poorly.
Gordon, it took me a bit of digging through various docs and updates on
elm-dev to understand it, but... the rational for deprecating elm-core/lazy
with memoization, is that it allowed recursive structures to be created on
the heap, and all other ways in which that was possible have been
https://gist.github.com/evancz/07436448b7d6c947f21742dab46d1218
It seems to be partly motivated by making garbage collection simpler -
presumably with WebAssembly in mind. I honestly don't know much of a real
advantage that is, but it seems that was the trade-off decision that has
already been made.
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N H
2017-11-27 00:22:24 UTC
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Basically, the way things work in Elm is that things exist if people need
them -- and can demonstrate the need. So far, there haven't been much need
for memoization, outside of the render loop. In fact, the main place where
lazy has been used at all has been for generating fuzzy values for
elm-test. There, I benchmarked and proved that elm-test is faster _without_
memoization. If you can identify a _real_ problem that you have that needs
implicit memoization at the function level (rather than the render level,
which elm-lang/virtual-dom provides) then make a case for it.
Post by W. Gordon Goodsman
Oh, I understand the rationale, but it limits the general usefulness of
lazy lists, as certain types of problems need memoization, and without Lazy
as it currently exists there is no way to accomplish it without JavaScript.
Just because the creators of the language don't see a need for it doesn't
mean it doesn't exist. Even for a lazy list of random numbers, no
memoization means that new random numbers will be computed every time the
list is scanned, meaning the lazy list won't appear to be static.
On Nov 20, 2017 17:35, "'Rupert Smith' via Elm Discuss" <
Post by 'Rupert Smith' via Elm Discuss
Post by W. Gordon Goodsman
The rational to depreciating these seems to be performance when applied
to their limited use in tests and I see that, as I tried to make changes to
Lazy that would improve its lack of performance when run on some JavaScript
engines that handle internal functions calling functions very poorly.
Gordon, it took me a bit of digging through various docs and updates on
elm-dev to understand it, but... the rational for deprecating elm-core/lazy
with memoization, is that it allowed recursive structures to be created on
the heap, and all other ways in which that was possible have been
https://gist.github.com/evancz/07436448b7d6c947f21742dab46d1218
It seems to be partly motivated by making garbage collection simpler -
presumably with WebAssembly in mind. I honestly don't know much of a real
advantage that is, but it seems that was the trade-off decision that has
already been made.
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Mark Hamburg
2017-11-27 07:49:54 UTC
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Example usage of laziness:

We have an expensive layout algorithm which depends both on the data being laid out and the view size. Whenever either of these changes, we need to re-run the layout algorithm but we would prefer to do the computation only when absolutely necessary and only once for any given set of parameters.

What keeps us from using lazy views for this is that the actual view instantiation optimizes based on the portion that is visible. If we used the view system to memoize the layout independent of the visible range then we couldn't introduce a dependency on the visible range below that point in the hierarchy.

Mark

P.S. Cyclic structures can be avoided by having the compiler perform a strongly connected component analysis (e.g., using Tarjan's algorithm) and disallowing any SCC's that include anything other than function definitions. In fact, not doing so likely leaves open other cyclic cases and hence getting rid of Lazy likely does not eliminate cycles and instead just narrows the cases for which Elm is suitable.
Basically, the way things work in Elm is that things exist if people need them -- and can demonstrate the need. So far, there haven't been much need for memoization, outside of the render loop. In fact, the main place where lazy has been used at all has been for generating fuzzy values for elm-test. There, I benchmarked and proved that elm-test is faster _without_ memoization. If you can identify a _real_ problem that you have that needs implicit memoization at the function level (rather than the render level, which elm-lang/virtual-dom provides) then make a case for it.
Oh, I understand the rationale, but it limits the general usefulness of lazy lists, as certain types of problems need memoization, and without Lazy as it currently exists there is no way to accomplish it without JavaScript.
Just because the creators of the language don't see a need for it doesn't mean it doesn't exist. Even for a lazy list of random numbers, no memoization means that new random numbers will be computed every time the list is scanned, meaning the lazy list won't appear to be static.
Post by 'Rupert Smith' via Elm Discuss
The rational to depreciating these seems to be performance when applied to their limited use in tests and I see that, as I tried to make changes to Lazy that would improve its lack of performance when run on some JavaScript engines that handle internal functions calling functions very poorly.
https://gist.github.com/evancz/07436448b7d6c947f21742dab46d1218
It seems to be partly motivated by making garbage collection simpler - presumably with WebAssembly in mind. I honestly don't know much of a real advantage that is, but it seems that was the trade-off decision that has already been made.
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'Rupert Smith' via Elm Discuss
2017-11-27 10:37:27 UTC
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Post by Mark Hamburg
P.S. Cyclic structures can be avoided by having the compiler perform a
strongly connected component analysis (e.g., using Tarjan's algorithm) and
disallowing any SCC's that include anything other than function
definitions. In fact, not doing so likely leaves open other cyclic cases
and hence getting rid of Lazy likely does not eliminate cycles and instead
just narrows the cases for which Elm is suitable.
My hunch is that enough has been done to make cyclic structures impossible,
but I have made no detailed analysis to support that. If all data
structures are immutable, then references within them have to be created
when the structure is first created. The first structure in a cycle to be
created must contain a reference to another structure in the cycle, for a
cycle to be formed, but as the other structures have not yet been created,
this cannot be done.

I think you should post an example of building a cyclic structure here, if
you think it can be done:

https://gist.github.com/evancz/07436448b7d6c947f21742dab46d1218
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Mark Hamburg
2017-11-27 16:33:46 UTC
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That page already has an example built using the decoder APIs so unless something is changing to disallow the creation of such cyclic decoders, cycles remain. The lifting trick still has a cycle at the end — it has to given the desire to build something recursive —through the functions and variables even if it doesn't end up in some of the data structures.

Specifically, since the lifting is being done to prevent repeated allocations of the Random.andThen and we potentially need to pass through the Random.andThen logic multiple times, the node must lead back to itself.

Mark
Post by Mark Hamburg
P.S. Cyclic structures can be avoided by having the compiler perform a strongly connected component analysis (e.g., using Tarjan's algorithm) and disallowing any SCC's that include anything other than function definitions. In fact, not doing so likely leaves open other cyclic cases and hence getting rid of Lazy likely does not eliminate cycles and instead just narrows the cases for which Elm is suitable.
My hunch is that enough has been done to make cyclic structures impossible, but I have made no detailed analysis to support that. If all data structures are immutable, then references within them have to be created when the structure is first created. The first structure in a cycle to be created must contain a reference to another structure in the cycle, for a cycle to be formed, but as the other structures have not yet been created, this cannot be done.
https://gist.github.com/evancz/07436448b7d6c947f21742dab46d1218
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'Rupert Smith' via Elm Discuss
2017-11-27 16:45:33 UTC
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Post by Mark Hamburg
That page already has an example built using the decoder APIs so unless
something is changing to disallow the creation of such cyclic decoders,
cycles remain.
I'm looking at the page. I don't see a cyclic example built with the
Decoder API. What am I missing?
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Mark Hamburg
2017-11-27 21:06:29 UTC
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My bad. Cyclic generator not decoder. Recursive decoders have the same issues.

Mark
Post by Mark Hamburg
That page already has an example built using the decoder APIs so unless something is changing to disallow the creation of such cyclic decoders, cycles remain.
I'm looking at the page. I don't see a cyclic example built with the Decoder API. What am I missing?
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'Rupert Smith' via Elm Discuss
2017-11-30 10:29:42 UTC
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Post by Mark Hamburg
Recursive decoders have the same issues.
I'll see if I can get my head around that. I use recursive decoders to
decode JSON into recursive types, but the values I output from this are not
recursive. I did not realize it is possible to build a recursive value with
decoders? Do you have an example?

These are mutually recursive types from a content model that I have been
working with. If the server sends me some content with a list of
relationships, and one of the relationships refers to the original content,
I will end up with multiple copies of the original content in the structure
decoded into Elm.

I need to use Decode.lazy to do this. Is that memoized? Is that how
decoders can create recursive values? Other than that, I don't see how they
could.

type Content =
Content
{
slug : Maybe String
, path : Maybe String
, contentType : Maybe (ContentType)
, model : ContentModel
, relationships : Maybe (List Relationship)
, container : Maybe (List Content)
, id : Maybe String
}


type Relationship =
Relationship
{
subject : Maybe (Content)
, predicate : Maybe PredicateType
, object : Maybe (Content)
, id : Maybe String
}
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'Rupert Smith' via Elm Discuss
2017-11-30 10:43:40 UTC
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Post by 'Rupert Smith' via Elm Discuss
I need to use Decode.lazy to do this. Is that memoized? Is that how
decoders can create recursive values? Other than that, I don't see how they
could.
Do you need to start with a recursive object in Javascript, then decode
into Elm using Decode.lazy, to produce a recursive value in Elm?

Approximately:

var rec = { field : rec };

myapp.ports.myport.send(rec);



type Rec =

Rec
{
field : Rec
}


port myport : (Rec -> msg) -> Sub msg



But of course, that isn't going to compile as ports won't decode tagged
union types, and you need a tagged union type to define a recursive type.
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Mark Hamburg
2017-12-05 17:16:15 UTC
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If you have a recursive JSON structure to decode, then the decoder either
needs to refer to itself (a cycle) or it needs to generate a new decoder on
each recursive step.

Mark


On Thu, Nov 30, 2017 at 2:43 AM, 'Rupert Smith' via Elm Discuss <
Post by 'Rupert Smith' via Elm Discuss
Post by 'Rupert Smith' via Elm Discuss
I need to use Decode.lazy to do this. Is that memoized? Is that how
decoders can create recursive values? Other than that, I don't see how they
could.
Do you need to start with a recursive object in Javascript, then decode
into Elm using Decode.lazy, to produce a recursive value in Elm?
var rec = { field : rec };
myapp.ports.myport.send(rec);
type Rec =
Rec
{
field : Rec
}
port myport : (Rec -> msg) -> Sub msg
But of course, that isn't going to compile as ports won't decode tagged
union types, and you need a tagged union type to define a recursive type.
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'Rupert Smith' via Elm Discuss
2017-12-07 10:32:44 UTC
Permalink
Post by Mark Hamburg
If you have a recursive JSON structure to decode, then the decoder either
needs to refer to itself (a cycle) or it needs to generate a new decoder on
each recursive step.
There is a difference between a function that is recursive and values on
the heap that are cyclic. The recursive functions issue seems to be with
what order to output compiled javascript code in, when there are mutually
recursive functions, as one may not be defined at the point in time when
another needs to access it. The cyclic structures on the heap issue, is to
do with how to make garbage collection very easy in a language that does
not need cyclic structures.

I think this page is confusing because it discusses both issues at the same
time:
https://gist.github.com/evancz/07436448b7d6c947f21742dab46d1218
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Mark Hamburg
2017-12-08 05:55:29 UTC
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Functions are also heap allocated objects and reference other objects — e.g., decoders — just like other objects do.

Mark
If you have a recursive JSON structure to decode, then the decoder either needs to refer to itself (a cycle) or it needs to generate a new decoder on each recursive step.
There is a difference between a function that is recursive and values on the heap that are cyclic. The recursive functions issue seems to be with what order to output compiled javascript code in, when there are mutually recursive functions, as one may not be defined at the point in time when another needs to access it. The cyclic structures on the heap issue, is to do with how to make garbage collection very easy in a language that does not need cyclic structures.
https://gist.github.com/evancz/07436448b7d6c947f21742dab46d1218
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'Rupert Smith' via Elm Discuss
2017-12-08 09:55:03 UTC
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Post by Mark Hamburg
Functions are also heap allocated objects and reference other objects —
e.g., decoders — just like other objects do.
But presumably functions do not get garbage collected?
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Mark Hamburg
2017-12-09 02:13:03 UTC
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Functions get garbage collected. Otherwise think what would happen every time you use partial function application or define a function within a let or other nested context. Because those functions (can) capture values, they are new values just as much as a list or record or other data value would be.

Mark
Post by 'Rupert Smith' via Elm Discuss
Post by Mark Hamburg
Functions are also heap allocated objects and reference other objects — e.g., decoders — just like other objects do.
But presumably functions do not get garbage collected?
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Mark Hamburg
2017-12-09 02:18:03 UTC
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Now if you narrow it to a module scope function and a module scope decoder or generator, then no they won't get collected anymore than anything else at module scope and hence the reference cycle is immaterial to the GC. But that argument would apply just as well to a cyclic list at module scope.

Mark
Post by Mark Hamburg
Functions get garbage collected. Otherwise think what would happen every time you use partial function application or define a function within a let or other nested context. Because those functions (can) capture values, they are new values just as much as a list or record or other data value would be.
Mark
Post by 'Rupert Smith' via Elm Discuss
Post by Mark Hamburg
Functions are also heap allocated objects and reference other objects — e.g., decoders — just like other objects do.
But presumably functions do not get garbage collected?
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'Rupert Smith' via Elm Discuss
2017-12-13 10:15:48 UTC
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Post by Mark Hamburg
Functions get garbage collected. Otherwise think what would happen every
time you use partial function application or define a function within a let
or other nested context. Because those functions (can) capture values, they
are new values just as much as a list or record or other data value would
be.
Is this something that is specific to the way javascript works? In my
experience partial function applications create a continuation, which is a
stack frame holding the captured values. Once the function completes the
stack frame is discarded, so there is no allocation or GC on the heap. Is
this not how Elm would be implemented on webasm and managing its own memory?
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Mark Hamburg
2017-12-16 21:27:18 UTC
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One can return a partial function application and it looks just like any other function, so in the general case, it can't be garbage collected. This isn't special to JavaScript. In fact, JavaScript doesn't support partial function application. What JavaScript, Elm, and pretty much any other language that purports to be functional does support is capturing values through closure creation and because those closures can be returned as values, they need to be subject to garbage collection.

Mark
Post by Mark Hamburg
Functions get garbage collected. Otherwise think what would happen every time you use partial function application or define a function within a let or other nested context. Because those functions (can) capture values, they are new values just as much as a list or record or other data value would be.
Is this something that is specific to the way javascript works? In my experience partial function applications create a continuation, which is a stack frame holding the captured values. Once the function completes the stack frame is discarded, so there is no allocation or GC on the heap. Is this not how Elm would be implemented on webasm and managing its own memory?
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'Rupert Smith' via Elm Discuss
2017-12-31 11:12:33 UTC
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Post by Mark Hamburg
One can return a partial function application and it looks just like any
other function, so in the general case, it can't be garbage collected. This
isn't special to JavaScript. In fact, JavaScript doesn't support partial
function application. What JavaScript, Elm, and pretty much any other
language that purports to be functional does support is capturing values
through closure creation and because those closures can be returned as
values, they need to be subject to garbage collection.
Thanks for this explanation, I get it now. I will try and work out an
example of how the mutually recursive functions can create a cyclic
structure on the heap, based on this.
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W. Gordon Goodsman
2017-12-06 02:38:03 UTC
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I agree that for your application in testing where memoization is not
required, your implementation is much faster. In fact, your new
implementation of a lazy list without memoization isn't really a lazy list
but rather a Co-Inductive Stream (CIS), where the use of a forcing function
is actually a nop and unnecessary, and the next Stream function could be
called directly.

I have used such CIS's effectively where memoization was not required and
had associated cost.

However, it seems to me to be flawed thinking to eliminate memoization from
the publically accessible libraries just because no one has yet used it.
There are classes of problems that are inefficient and unnecessarily repeat
calculations without memoization.

For example, the Hamming numbers sequence can be produced elegantly using
true memoized lazy lists in Haskell as:

hamming() = 1 : foldr [] [5,3,2] where
u n s = r where
r = merge s (map (n*) (1:r))

merge [] b = b
merge a@(x:xs) b@(y:ys)
| x < y = x : merge xs b
| otherwise = y : merge a ys

Using memoized lazy lists, this algorithm is almost as elegant and
efficient in Elm as in Haskell, but using only CIS's makes it both time and
space inefficient, with redundantly repeated calculations and the streams
unable to be garbage collected due to the functions needing to retain back
pointers into the streams all the way back to the beginning.

There are many similar examples in mathematics and perhaps in graphics
applications.

It seems to me to be a mistake to limit the generality of the language
libraries just because the authors don't see a use case. While it is true
that many JavaScript engines (all except Microsoft Edge) limit the
efficiency of how memoization must be implemented for Elm (a function
generating a function), this won't always necessarily be true, and almost
certainly won't be true if and when Elm generates wasm code, now supported
by all major browsers.

CIS's are easy enough to generate that they could be defined where needed
as for the testing case, or could be a separate library.

Meanwhile the memoized Lazy type library really is needed, else there is no
way to implement memoization without custom JavaScript, which is frowned
upon.
Post by N H
Basically, the way things work in Elm is that things exist if people need
them -- and can demonstrate the need. So far, there haven't been much need
for memoization, outside of the render loop. In fact, the main place where
lazy has been used at all has been for generating fuzzy values for
elm-test. There, I benchmarked and proved that elm-test is faster _without_
memoization. If you can identify a _real_ problem that you have that needs
implicit memoization at the function level (rather than the render level,
which elm-lang/virtual-dom provides) then make a case for it.
Post by W. Gordon Goodsman
Oh, I understand the rationale, but it limits the general usefulness of
lazy lists, as certain types of problems need memoization, and without Lazy
as it currently exists there is no way to accomplish it without JavaScript.
Just because the creators of the language don't see a need for it doesn't
mean it doesn't exist. Even for a lazy list of random numbers, no
memoization means that new random numbers will be computed every time the
list is scanned, meaning the lazy list won't appear to be static.
On Nov 20, 2017 17:35, "'Rupert Smith' via Elm Discuss" <
Post by 'Rupert Smith' via Elm Discuss
Post by W. Gordon Goodsman
The rational to depreciating these seems to be performance when applied
to their limited use in tests and I see that, as I tried to make changes to
Lazy that would improve its lack of performance when run on some JavaScript
engines that handle internal functions calling functions very poorly.
Gordon, it took me a bit of digging through various docs and updates on
elm-dev to understand it, but... the rational for deprecating elm-core/lazy
with memoization, is that it allowed recursive structures to be created on
the heap, and all other ways in which that was possible have been
https://gist.github.com/evancz/07436448b7d6c947f21742dab46d1218
It seems to be partly motivated by making garbage collection simpler -
presumably with WebAssembly in mind. I honestly don't know much of a real
advantage that is, but it seems that was the trade-off decision that has
already been made.
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'Rupert Smith' via Elm Discuss
2017-12-08 10:04:21 UTC
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Post by W. Gordon Goodsman
Meanwhile the memoized Lazy type library really is needed, else there is
no way to implement memoization without custom JavaScript, which is frowned
upon.
I think you make a good case for keeping it in Gordon. When I started this
thread, all I really needed is a CIS for random numbers, but I can see that
true lazy lists have their uses. Is it really worth throwing them out to
make garbage collection by reference counting only possible if/when Elm is
ported to web assembly?
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