# Merging disjoint NamedTuples

**URL:** <https://discourse.julialang.org/t/merging-disjoint-namedtuples/137650>\
**Category:** General Usage\
**Tags:** question, namedtuple\
**Created:** [June 16, 2026, 10:56am UTC](https://discourse.julialang.org/t/merging-disjoint-namedtuples/137650 "2026-06-16T10:56:02Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![Tamas\_Papp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamas_papp/32/25949_2.png) [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)\
**Post date:** [June 16, 2026, 10:56am UTC](https://discourse.julialang.org/t/merging-disjoint-namedtuples/137650/1 "2026-06-16T10:56:02Z")

</div>

I need a function that merges two `NamedTuple`s, but throws an error if they have elements in common. Not unlike

```julia
@generated function merge_disjoint(a::NamedTuple{A}, b::NamedTuple{B}) where {A,B}
    AB = intersect(A, B)
    if isempty(AB)
        :(merge(a, b))
    else
        msg = "found common names $(join(AB, ", "))"
        :(throw(ArgumentError($msg)))
    end
end

```

```julia
julia> ab = (a= 1, b = 2)
(a = 1, b = 2)

julia> merge_disjoint(ab, ab)
ERROR: ArgumentError: found common names a, b
Stacktrace:
 [1] macro expansion
   @ ./REPL[63]:1 [inlined]
 [2] merge_disjoint(a::@NamedTuple{a::Int64, b::Int64}, b::@NamedTuple{a::Int64, b::Int64})
   @ Main ./REPL[63]:1
 [3] top-level scope
   @ REPL[71]:1

julia> merge_disjoint(ab, (c = 1, d = 2))
(a = 1, b = 2, c = 1, d = 2)

```

I am wondering if there anything simpler than the above.

---

<div class="post-metadata">

**Author:** ![BLI](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bli/32/37206_2.png) [@BLI](https://discourse.julialang.org/u/BLI)\
**Post date:** [June 16, 2026, 11:24am UTC](https://discourse.julialang.org/t/merging-disjoint-namedtuples/137650/2 "2026-06-16T11:24:47Z")

</div>

My attempt is probably too simple… given that you use a macro, etc.

```julia-auto
function merge_disjoint(x,y)
    x_k = keys(x)
    y_k = keys(y)
    if length(intersect(x_k,y_k)) == 0
        merge(x,y)
    else
        println("Shared names")
    end
end

```

---

<div class="post-metadata">

**Author:** ![foobar\_lv2](https://avatars.discourse-cdn.com/v4/letter/f/ee59a6/32.png) [@foobar\_lv2](https://discourse.julialang.org/u/foobar_lv2)\
**Post date:** [June 16, 2026, 11:39am UTC](https://discourse.julialang.org/t/merging-disjoint-namedtuples/137650/3 "2026-06-16T11:39:05Z")

</div>

What about

```julia-auto
julia> function mergeDisjoint(a,b)
       res = merge(a,b)
       length(res) == length(a) + length(b) ? res : throw("oops")
       end

```

Relevant parts:

1. You don’t need to be @generated – merge itself is already generated
2. You don’t need to compute the intersection, you can simply check lengths
3. Method specialization means that the issue is visible from the stacktrace anyways:

```julia-auto
julia> mergeDisjoint((;a=1, b=2), (;c=3, b=1))
ERROR: "oops"
Stacktrace:
 [1] mergeDisjoint(a::@NamedTuple{a::Int64, b::Int64}, b::@NamedTuple{c::Int64, b::Int64})
   @ Main ./REPL[39]:3
 [2] top-level scope
   @ REPL[41]:1

```

1. This solution correctly infers (to either `Union{}` in case of error, or the correct return-value).

---

<div class="post-metadata">

**Author:** ![Tamas\_Papp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamas_papp/32/25949_2.png) [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)\
**Post date:** [June 16, 2026, 1:10pm UTC](https://discourse.julialang.org/t/merging-disjoint-namedtuples/137650/4 "2026-06-16T13:10:54Z")

</div>

Thanks! I will use that as a solution, slightly tweaked as I want an informative error message:

```julia
function merge_disjoint(a::NamedTuple, b::NamedTuple)
    ab = merge(a, b)
    if length(a) + length(b) ≠ length(ab)
        throw(ArgumentError("found common keys " *
            join(intersect(keys(a), keys(b)), ", ")))
    end
    ab
end

```