# How to avoid full specialization of \`haskey(::Dict, key)\` on the caller side

**URL:** <https://discourse.julialang.org/t/how-to-avoid-full-specialization-of-haskey-dict-key-on-the-caller-side/67323>\
**Category:** New to Julia\
**Tags:** question, compilation\
**Created:** [August 30, 2021, 2:31am UTC](https://discourse.julialang.org/t/how-to-avoid-full-specialization-of-haskey-dict-key-on-the-caller-side/67323 "2021-08-30T02:31:49Z")\
**Posts on this page:** 7\
**Page:** 1

<div class="post-metadata">

**Author:** ![kimikage](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kimikage/32/14534_2.png) [@kimikage](https://discourse.julialang.org/u/kimikage)\
**Post date:** [August 30, 2021, 2:31am UTC](https://discourse.julialang.org/t/how-to-avoid-full-specialization-of-haskey-dict-key-on-the-caller-side/67323/1 "2021-08-30T02:31:50Z")

</div>

I don’t understand the details of the implementation of `Dict`, but I don’t think the `V` in `Dict{K,V}` is involved in the “essential functionality” in `haskey`.  
For example, when I start a REPL session, there are method instances such as `haskey(::Dict{String, ::Any}, ::String)`. However, within ordinary call of `haskey`, full specialization will be done. Is there an easy way to avoid the full specialization on the caller side?

```julia
julia> dict = Dict("key" => (1, 2))
Dict{String, Tuple{Int64, Int64}} with 1 entry:
  "key" => (1, 2)

julia> using MethodAnalysis

julia> methodinstance(Tuple{typeof(haskey), Dict{String, Any}, String}) # just for example
MethodInstance for haskey(::Dict{String, Any}, ::String)

julia> methodinstance(Tuple{typeof(haskey), Dict{String, Tuple{Int, Int}}, String})

julia> haskey(dict, "key")
true

julia> methodinstance(Tuple{typeof(haskey), Dict{String, Tuple{Int, Int}}, String})
MethodInstance for haskey(::Dict{String, Tuple{Int64, Int64}}, ::String)

julia> versioninfo()
Julia Version 1.8.0-DEV.442
Commit 897c8e607f (2021-08-29 18:44 UTC)
Platform Info:
  OS: Windows (x86_64-w64-mingw32)
  CPU: 11th Gen Intel(R) Core(TM) i7-1165G7 @ 2.80GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-12.0.1 (ORCJIT, tigerlake)

```

Of course, I know that:

```julia
julia> Dict{String, Tuple{Int, Int}} <: Dict{String, Any} 
false

julia> Dict{String, Tuple{Int, Int}} <: Dict{String}
true

```

So, I thought the following method would be helpful, but in reality it was not.

```julia
_haskey(@nospecialize(dict::Dict{String}), key::String) = haskey(dict, key)

```

I picked up the example of `haskey` as a simple case, and “I” don’t have any performance troubles with `haskey` itself. I am more interested in more general solutions than the `Dict` specific techniques.

---

<div class="post-metadata">

**Author:** ![anon56330260](https://avatars.discourse-cdn.com/v4/letter/a/f07891/32.png) [@anon56330260](https://discourse.julialang.org/u/anon56330260)\
**Post date:** [August 30, 2021, 3:55am UTC](https://discourse.julialang.org/t/how-to-avoid-full-specialization-of-haskey-dict-key-on-the-caller-side/67323/2 "2021-08-30T03:55:24Z")

</div>

`Dict{K,V}` is essentially a `Vector{Tuple{Int,K,V}}`, where `Int` is the hash value of `K`. So to get the value from the vector, you must know the size of `V`, which requires the specialization of `V`.

---

<div class="post-metadata">

**Author:** ![kimikage](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kimikage/32/14534_2.png) [@kimikage](https://discourse.julialang.org/u/kimikage)\
**Post date:** [August 30, 2021, 4:13am UTC](https://discourse.julialang.org/t/how-to-avoid-full-specialization-of-haskey-dict-key-on-the-caller-side/67323/3 "2021-08-30T04:13:22Z")

</div>

That’s a good point. However, I think it’s about the internal implementation and not the interface.

Can’t we be a little more flexible in our use of dynamic dispatching?

(As an aside, I am skeptical about the hash, key, and value being tightly coupled.)

---

<div class="post-metadata">

**Author:** ![anon56330260](https://avatars.discourse-cdn.com/v4/letter/a/f07891/32.png) [@anon56330260](https://discourse.julialang.org/u/anon56330260)\
**Post date:** [August 30, 2021, 6:24am UTC](https://discourse.julialang.org/t/how-to-avoid-full-specialization-of-haskey-dict-key-on-the-caller-side/67323/4 "2021-08-30T06:24:35Z")

</div>

I mistake the implementation of `Dict`. It actually stores values and keys separately (so it’s `Vector{K}` and `Vector{V}`). I have another fast implementation which stores them together. So you are right. It doesn’t depend on `V`.

But at least in my case, `V` is implicitly used, so we have to specialize on V.

---

<div class="post-metadata">

**Author:** ![anon56330260](https://avatars.discourse-cdn.com/v4/letter/a/f07891/32.png) [@anon56330260](https://discourse.julialang.org/u/anon56330260)\
**Post date:** [August 30, 2021, 6:33am UTC](https://discourse.julialang.org/t/how-to-avoid-full-specialization-of-haskey-dict-key-on-the-caller-side/67323/5 "2021-08-30T06:33:16Z")

</div>

Can you try `_haskey(@nospecialize(dict::Dict{String,T} where T), key::String) = haskey(dict, key)`, it seems this is not specialized over different `T`.

---

<div class="post-metadata">

**Author:** ![kimikage](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kimikage/32/14534_2.png) [@kimikage](https://discourse.julialang.org/u/kimikage)\
**Post date:** [August 30, 2021, 6:48am UTC](https://discourse.julialang.org/t/how-to-avoid-full-specialization-of-haskey-dict-key-on-the-caller-side/67323/6 "2021-08-30T06:48:49Z")

</div>

Unfortunately, it doesn’t work, as the last three lines show. 😕

```julia
julia> dict = Dict("key" => (1, 2))
Dict{String, Tuple{Int64, Int64}} with 1 entry:
  "key" => (1, 2)

julia> using MethodAnalysis

julia> _haskey(@nospecialize(dict::Dict{String,T} where T), key::String) = haskey(dict, key)
_haskey (generic function with 1 method)

julia> _haskey(dict, "key")
true

julia> foreach(x->occursin("(::Dict{String", string(x)) && println(x), methodinstances(haskey))
MethodInstance for haskey(::Dict{String, Base.CachedTOMLDict}, ::String)
MethodInstance for haskey(::Dict{String, Nothing}, ::String)
MethodInstance for haskey(::Dict{String, Any}, ::String)
MethodInstance for haskey(::Dict{String, Pkg.REPLMode.OptionSpec}, ::String)
MethodInstance for haskey(::Dict{String, Pkg.REPLMode.CommandSpec}, ::String)
MethodInstance for haskey(::Dict{String, Dict{String, Pkg.REPLMode.CommandSpec}}, ::String)
MethodInstance for haskey(::Dict{String, String}, ::String)
MethodInstance for haskey(::Dict{String, Function}, ::String)
MethodInstance for haskey(::Dict{String, LibGit2.AbstractCredential}, ::String)
MethodInstance for haskey(::Dict{String, Pkg.Types.Compat}, ::String)
MethodInstance for haskey(::Dict{String, Pkg.Versions.VersionSpec}, ::String)
MethodInstance for haskey(::Dict{String, Base.UUID}, ::String)
MethodInstance for haskey(::Dict{String, Bool}, ::Nothing)
MethodInstance for haskey(::Dict{String, Bool}, ::String)
MethodInstance for haskey(::Dict{String, Tuple{Int64, Int64}}, ::String)
MethodInstance for haskey(::Dict{String}, ::String)
MethodInstance for haskey(::Dict{String, <:Function}, ::String)

```

---

<div class="post-metadata">

**Author:** ![anon56330260](https://avatars.discourse-cdn.com/v4/letter/a/f07891/32.png) [@anon56330260](https://discourse.julialang.org/u/anon56330260)\
**Post date:** [August 30, 2021, 10:56am UTC](https://discourse.julialang.org/t/how-to-avoid-full-specialization-of-haskey-dict-key-on-the-caller-side/67323/7 "2021-08-30T10:56:36Z")

</div>

I think it’s impossible to do this using ordinary Julia (I suspect that even you break into Julia’s internal, you still can’t do it). The basic idea here is to invoke a specific method instance of `haskey` (namely `haskey(::{Dict{String,T} where T, ::String})` and that specific method instance requires special compilation (to avoid specialization over different input types, otherwise it will still call JIT at runtime).

It’s strange that this feature doesn’t get implemented in Julia, since Julia can already infer type of `haskey` when input type is abstract. I always thought Julia can do this before, but it seems that this is not the case.