# Accessing the field of \`Pair\` seems to be type-unstable

**URL:** <https://discourse.julialang.org/t/accessing-the-field-of-pair-seems-to-be-type-unstable/78045>\
**Category:** General Usage\
**Tags:** type-stability\
**Created:** [March 17, 2022, 6:12pm UTC](https://discourse.julialang.org/t/accessing-the-field-of-pair-seems-to-be-type-unstable/78045 "2022-03-17T18:12:25Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![frankwswang](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/frankwswang/32/18561_2.png) [@frankwswang](https://discourse.julialang.org/u/frankwswang)\
**Post date:** [March 17, 2022, 6:12pm UTC](https://discourse.julialang.org/t/accessing-the-field-of-pair-seems-to-be-type-unstable/78045/1 "2022-03-17T18:12:25Z")

</div>

MWE (Julia v1.6 & v1.7):

```julia
julia> p1 = Pair(:a, 2)
:a => 2

julia> @code_warntype p1.first
Variables
  #self#::Core.Const(getproperty)
  x::Pair{Symbol, Int64}
  f::Symbol

Body::Union{Int64, Symbol}
1 ─ %1 = Base.getfield(x, f)::Union{Int64, Symbol}
└── return %1

julia> @code_warntype p1[1]
Variables
  #self#::Core.Const(getindex)
  p::Pair{Symbol, Int64}
  i::Int64

Body::Union{Int64, Symbol}
1 ─ %1 = Base.getfield(p, i)::Union{Int64, Symbol}
└── return %1

julia> @code_warntype p1.second
Variables
  #self#::Core.Const(getproperty)
  x::Pair{Symbol, Int64}
  f::Symbol

Body::Union{Int64, Symbol}
1 ─ %1 = Base.getfield(x, f)::Union{Int64, Symbol}
└── return %1

julia> @code_warntype p1[2]
Variables
  #self#::Core.Const(getindex)
  p::Pair{Symbol, Int64}
  i::Int64

Body::Union{Int64, Symbol}
1 ─ %1 = Base.getfield(p, i)::Union{Int64, Symbol}
└── return %1

```

Is there a reason for this result and can’t it be designed to be type-stable? Thanks!

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<div class="post-metadata">

**Author:** ![fredrikekre](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/fredrikekre/32/1688_2.png) [@fredrikekre](https://discourse.julialang.org/u/fredrikekre)\
**Post date:** [March 17, 2022, 6:24pm UTC](https://discourse.julialang.org/t/accessing-the-field-of-pair-seems-to-be-type-unstable/78045/2 "2022-03-17T18:24:55Z")

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You need to do introspection like this in a function:

```julia
julia> p = Pair(:a, 2);

julia> f(x) = x.first;

julia> @code_warntype f(p)
MethodInstance for f(::Pair{Symbol, Int64})
  from f(x) in Main at REPL[5]:1
Arguments
  #self#::Core.Const(f)
  x::Pair{Symbol, Int64}
Body::Symbol
1 ─ %1 = Base.getproperty(x, :first)::Symbol
└── return %1

```

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<div class="post-metadata">

**Author:** ![frankwswang](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/frankwswang/32/18561_2.png) [@frankwswang](https://discourse.julialang.org/u/frankwswang)\
**Post date:** [March 17, 2022, 6:28pm UTC](https://discourse.julialang.org/t/accessing-the-field-of-pair-seems-to-be-type-unstable/78045/3 "2022-03-17T18:28:26Z")

</div>

Thanks! Do you know why if I don’t wrap it in a function it is shown to be type-unstable?

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<div class="post-metadata">

**Author:** ![rdeits](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rdeits/32/286_2.png) [@rdeits](https://discourse.julialang.org/u/rdeits)\
**Post date:** [March 17, 2022, 6:30pm UTC](https://discourse.julialang.org/t/accessing-the-field-of-pair-seems-to-be-type-unstable/78045/4 "2022-03-17T18:30:29Z")

</div>

To elaborate, `@code_warntype` only sees the _types_ of the arguments to the function call you give it.  
The syntax `p1.first` is just a shorthand for `getproperty(p1, :first)`, so when you do: `@code_warntype p1.first`, you’re seeing the result of:

```julia
code_warntype(getproperty, (typeof(p1), typeof(:first)))

```

or, in other words:

```julia
code_warntype(getproperty, (Pair{Symbol, Int}, Symbol))

```

which isn’t enough information to indicate whether you’re accessing the first or second element.

Putting the access into a function lets Julia’s usual constant propagation machinery do its work, and `@code_warntype` can correctly show you that result when you use it on the new function.