# Caching function values

**URL:** <https://discourse.julialang.org/t/caching-function-values/2092>\
**Category:** General Usage\
**Tags:** question\
**Created:** [February 14, 2017, 12:39am UTC](https://discourse.julialang.org/t/caching-function-values/2092 "2017-02-14T00:39:10Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![lostella](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lostella/32/356_2.png) [@lostella](https://discourse.julialang.org/u/lostella)\
**Post date:** [February 14, 2017, 12:39am UTC](https://discourse.julialang.org/t/caching-function-values/2092/1 "2017-02-14T00:39:10Z")

</div>

Hello,

This may be silly. I would like to understand whether this is a viable design pattern in Julia. I want to define a new type `GraphPoint <: AbstractArray` that joins together a point and a function value at that point (like a cache, to avoid re-evaluating a function if you will). I would do it pretty much like this:

```julia
immutable GraphPoint <: AbstractArray
  x::AbstractArray
  fx::Real
end

```

The idea is to have a function that uses this information to avoid computations, as in

```julia
function myFun(point::GraphPoint)
  return x.fx
end

function myFun(x::AbstractArray)
  # compute something with x
  return something
end

```

`GraphPoint` objects are created somewhere. It’s not relevant by whom or when. The point is, I would like to naturally extend all _other_ functions defined on `AbstractArray` to my new type `GraphPoint`, by simply redirecting them to the `x` field of the structure. Say, I would like to do matrix-vector products with `A::AbstractMatrix` and an object `point::GraphPoint` by doing `A*point.x`.

Is there a way to naturally extend _all_ functions defined on `AbstractArrat` to my new type `GraphPoint` in this sense?

Thank you.

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<div class="post-metadata">

**Author:** ![tim.holy](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tim.holy/32/52_2.png) [@tim.holy](https://discourse.julialang.org/u/tim.holy)\
**Post date:** [February 15, 2017, 1:41pm UTC](https://discourse.julialang.org/t/caching-function-values/2092/2 "2017-02-15T13:41:43Z")

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This is called “memoization.” If it helps, there’s already a package: [https://github.com/simonster/Memoize.jl](https://github.com/simonster/Memoize.jl).

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<div class="post-metadata">

**Author:** ![lostella](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lostella/32/356_2.png) [@lostella](https://discourse.julialang.org/u/lostella)\
**Post date:** [February 15, 2017, 5:32pm UTC](https://discourse.julialang.org/t/caching-function-values/2092/3 "2017-02-15T17:32:06Z")

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Wow, that’s really interesting, thank you. I will definitely look into it. The reason why I was thinking of a structure like the one I described, is that in my case the value of the function may be obtained somewhere else than by calling the function itself: in that instant, I would like to store such value along with the point.

It is possible though that I could combine memoization into my scenario, to obtain what I need.

Thank you again!

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<div class="post-metadata">

**Author:** ![tim.holy](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tim.holy/32/52_2.png) [@tim.holy](https://discourse.julialang.org/u/tim.holy)\
**Post date:** [February 16, 2017, 10:34pm UTC](https://discourse.julialang.org/t/caching-function-values/2092/4 "2017-02-16T22:34:22Z")

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Sure, if you’ll need that value in many places, it’s entirely reasonable to carry it around. Good luck!

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<div class="post-metadata">

**Author:** ![cstjean](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/cstjean/32/1444_2.png) [@cstjean](https://discourse.julialang.org/u/cstjean)\
**Post date:** [February 17, 2017, 3:28am UTC](https://discourse.julialang.org/t/caching-function-values/2092/5 "2017-02-17T03:28:20Z")

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> [@lostella](#):
>
> Is there a way to naturally extend all functions defined on AbstractArrat to my new type GraphPoint in this sense?

Not that I know of, but there are various delegation macros floating around that have a similar effect. I use

```julia
""" `delegate(f, typfield)` delegates `f(::T, args...)` to `f(t.field, args...)`

    type AA
        a
    end

    @delegate(Base.length, AA.a)
    length(AA([1,2,3])) # -> 3
"""
macro delegate(f, typfield)
    @assert typfield.head == :.
    typ = typfield.args[1]
    field = typfield.args[2].args[1]
    :(function $(esc(f))(obj::$(esc(typ)), args...; kwargs...)
      # Not sure why, but using @inline caused an error on Julia 0.4
      $(esc(Expr(:meta, :inline)))
      $(esc(f))(getfield(obj, $(esc(Expr(:quote, field)))), args...; kwargs...)
    end)
end

```

Memoize.jl might work, but AFAIK it doesn’t use weak-key dictionaries, so it’ll keep your objects arrays until you explicitly clear the memo. That’s not a problem with the GraphPoint.fx solution (I would use a Nullable there)