# Make \`cat\` (on a fixed dimension) type-stable
**URL:** https://discourse.julialang.org/t/make-cat-on-a-fixed-dimension-type-stable/15183
**Category:** General Usage
**Created:** [September 19, 2018, 4:38pm UTC](https://discourse.julialang.org/t/make-cat-on-a-fixed-dimension-type-stable/15183 "2018-09-19T16:38:15Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@kli6891](https://discourse.julialang.org/u/kli6891)
#### Post date: [September 19, 2018, 4:38pm UTC](https://discourse.julialang.org/t/make-cat-on-a-fixed-dimension-type-stable/15183/1 "2018-09-19T16:38:15Z")
I posted on Slack but wasn’t able to get a resolution (my fault since I posted at 4AM EST and Slack is so fast moving).
Anyhow, is there a way to make `cat` type stable? I see the following:
```julia
A = Array{Float64, 3}(reshape(1:2*2*3, (2,2,3)));
@code_warntype cat(zeros(2,2), A; dims=3)
```
which gives me
```julia
Body::Any
1 ── %1 = (Base.getfield)(#temp#, :dims)::Int64 │╻ getindex
│ %2 = (Base.slt_int)(0, 1)::Bool ││╻╷╷╷ iterate
└─── goto #3 if not %2 │││┃│ iterate
2 ── goto #4 ││││┃ iterate
3 ── invoke Base.getindex(()::Tuple{}, 1::Int64) │││││
└─── $(Expr(:unreachable)) │││││
4 ┄─ goto #5 ││││
5 ── goto #6 ││╻ iterate
6 ── goto #7 ││
7 ── nothing │
│ %11 = (getfield)(A, 1)::Array{Float64,2} │
│ %12 = (getfield)(A, 2)::Array{Float64,3} │
│ invoke Core.kwfunc(Base.cat_t::Any) ││╻ _cat
│ %14 = (Base.slt_int)(0, 1)::Bool │││╻╷╷╷╷ #cat_t
└─── goto #9 if not %14 ││││┃│││ isempty
8 ── goto #10 │││││┃││ iterate
9 ── invoke Base.getindex(()::Tuple{}, 1::Int64) ││││││┃│ iterate
└─── $(Expr(:unreachable)) │││││││┃ iterate
10 ┄ goto #11 │││││││
11 ─ goto #12 │││││╻ iterate
12 ─ goto #13 │││││
13 ─ %22 = invoke Base._cat_t(%1::Int64, Float64::Type, %11::Array{Float64,2}, %12::Vararg{Any,N} where N)::Any ││││╻ #cat_t#99
└─── goto #14 ││││
14 ─ goto #15 │││
15 ─ goto #16
```
* * *
Curiously, even though `vcat(x...)` is semantically the same as `cat(x...; dims=1)`, the latter **is** type stable:
```julia
julia> @code_warntype cat(zeros(2,2,3), A; dims=1)
Body::Any
1 ── %1 = (Base.getfield)(#temp#, :dims)::Int64 │╻ getindex
│ %2 = (Base.slt_int)(0, 1)::Bool ││╻╷╷╷ iterate
└─── goto #3 if not %2 │││┃│ iterate
2 ── goto #4 ││││┃ iterate
3 ── invoke Base.getindex(()::Tuple{}, 1::Int64) │││││
└─── $(Expr(:unreachable)) │││││
4 ┄─ goto #5 ││││
5 ── goto #6 ││╻ iterate
6 ── goto #7 ││
7 ── nothing │
│ %11 = (getfield)(A, 1)::Array{Float64,3} │
│ %12 = (getfield)(A, 2)::Array{Float64,3} │
│ invoke Core.kwfunc(Base.cat_t::Any) ││╻ _cat
│ %14 = (Base.slt_int)(0, 1)::Bool │││╻╷╷╷╷ #cat_t
└─── goto #9 if not %14 ││││┃│││ isempty
8 ── goto #10 │││││┃││ iterate
9 ── invoke Base.getindex(()::Tuple{}, 1::Int64) ││││││┃│ iterate
└─── $(Expr(:unreachable)) │││││││┃ iterate
10 ┄ goto #11 │││││││
11 ─ goto #12 │││││╻ iterate
12 ─ goto #13 │││││
13 ─ %22 = invoke Base._cat_t(%1::Int64, Float64::Type, %11::Array{Float64,3}, %12::Vararg{Array{Float64,3},N} where N)::Any╻ #cat_t#99
└─── goto #14 ││││
14 ─ goto #15 │││
15 ─ goto #16 ││
16 ─ return %22 │
```
versus
```julia
julia> @code_warntype vcat(zeros(2,2,3), A)
Body::Array{Float64,3}
1486 1 ─ invoke Core.kwfunc(Base.cat::Any) │
│ %2 = (Base.slt_int)(0, 1)::Bool │╻╷╷╷╷ #cat
└── goto #3 if not %2 ││┃│││ isempty
2 ─ goto #4 │││┃││ iterate
3 ─ invoke Base.getindex(()::Tuple{}, 1::Int64) ││││┃│ iterate
└── $(Expr(:unreachable)) │││││┃ iterate
4 ┄ goto #5 │││││
5 ─ goto #6 │││╻ iterate
6 ─ goto #7 │││
7 ─ %10 = %new(NamedTuple{(:dims,),Tuple{Val{1}}}, $(QuoteNode(Val{1}())))::NamedTuple{(:dims,),Tuple{Val{1}}} │││╻╷╷╷ _cat
│ %11 = invoke Core.kwfunc(Base.cat_t::Any)::getfield(Base, Symbol("#kw##cat_t")) ││││
│ %12 = Base.cat_t::typeof(Base.cat_t) ││││
│ %13 = invoke %11(%10::NamedTuple{(:dims,),Tuple{Val{1}}}, %12::typeof(Base.cat_t), Float64::Type{Float64}, _2::Array{Float64,3}, _3::Vararg{Array{Float64,3},N} where N)::Array{Float64,3}
└── goto #8 ││
8 ─ return %13 │
```
What is going on here?
---
### Author:  [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)
#### Post date: [September 19, 2018, 4:46pm UTC](https://discourse.julialang.org/t/make-cat-on-a-fixed-dimension-type-stable/15183/2 "2018-09-19T16:46:35Z")
`1` is a runtime value and (modulu constant propagation) type inference deals with types. So you get a different type returned based on the runtime value of `dims`.
---
### Author:  [@mbauman](https://discourse.julialang.org/u/mbauman)
#### Post date: [September 19, 2018, 5:05pm UTC](https://discourse.julialang.org/t/make-cat-on-a-fixed-dimension-type-stable/15183/3 "2018-09-19T17:05:05Z")
Slightly more info: `@code_warntype` assumes that its arguments are not constant… but often you’ll write the dims keyword as a constant. You can see this behavior simply by introspecting a simple wrapper function where you’re written it as the literal `3`:
```julia
julia> cat3(args...) = cat(args...; dims=3)
cat3 (generic function with 1 method)
julia> @code_warntype cat3(zeros(2,2), A)
Body::Array{Float64,3}
…
```
---
### Author:  [@kli6891](https://discourse.julialang.org/u/kli6891)
#### Post date: [September 19, 2018, 5:09pm UTC](https://discourse.julialang.org/t/make-cat-on-a-fixed-dimension-type-stable/15183/5 "2018-09-19T17:09:03Z")
Thanks for the help! The answers make a lot of sense. @mbauman I noticed the type instability in a function, where I used `dims=3` with `cat`. How should I understand this, given your example?
Specifically, my code is:
```julia
function addones(x :: Array{T, 3} where T <: Real)
shape = (size(x, 1), size(x, 2))
return cat(ones(shape), x; dims=3)
end
```
---
### Author:  [@improbable22](https://discourse.julialang.org/u/improbable22)
#### Post date: [September 19, 2018, 7:11pm UTC](https://discourse.julialang.org/t/make-cat-on-a-fixed-dimension-type-stable/15183/6 "2018-09-19T19:11:25Z")
This function doesn’t seem to be type-stable for me (on 0.7):
```julia
julia> @code_warntype cat3(zeros(2,2), A)
Body::Any
```
What is stable however is using `dims=Val(3)`:
```julia
julia> @code_warntype cat(zeros(2,2), A; dims=Val(3))
Body::Array{Float64,3}
```