# Type inference when generating a NamedTuple
**URL:** https://discourse.julialang.org/t/type-inference-when-generating-a-namedtuple/16222
**Category:** General Usage
**Tags:** question
**Created:** [October 12, 2018, 8:55am UTC](https://discourse.julialang.org/t/type-inference-when-generating-a-namedtuple/16222 "2018-10-12T08:55:36Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [October 12, 2018, 8:55am UTC](https://discourse.julialang.org/t/type-inference-when-generating-a-namedtuple/16222/1 "2018-10-12T08:55:36Z")
This is an MWE simplified from a real-world problem — I have a `T <: NamedTuple` _type_, and would like to generate an _value_ `::T` from functions that generate fields based on type, to be named appropriately.
```julia
using Random
struct Gen{T <: NamedTuple, R <: AbstractRNG}
rng::R # the original problem has a mutable field capturing a state, too
end
# convenience constructor
Gen{T}(rng::R) where {T <: NamedTuple, R <: AbstractRNG} = Gen{T,R}(rng)
# field generator: in the MWE, random
genfield(rng::AbstractRNG, ::Type{T}) where T <: Real = rand(rng, T)
# field generator for potentially missing
genfield(rng::AbstractRNG, ::Type{Union{Missing, T}}) where T =
rand(rng) < 0.5 ? missing : genfield(rng, T)
# THIS is the problematic function
gen(g::Gen{T}) where T =
T(ntuple(i -> genfield(g.rng, fieldtype(T, i)), fieldcount(T)))
g = Gen{NamedTuple{(:a, :b), Tuple{Int, Union{Missing, Float64}}}}(Random.GLOBAL_RNG)
@code_warntype genfield(Random.GLOBAL_RNG, Union{Missing, Float64})
@code_warntype gen(g)
```
With lot of the output cut,
```julia
julia> @code_warntype genfield(Random.GLOBAL_RNG, Union{Missing, Float64})
Body::Union{Missing, Float64}
julia> @code_warntype gen(g)
Body::Any
julia> VERSION
v"1.1.0-DEV.439"
```
---
### Author:  [@jandehaan](https://discourse.julialang.org/u/jandehaan)
#### Post date: [October 12, 2018, 2:32pm UTC](https://discourse.julialang.org/t/type-inference-when-generating-a-namedtuple/16222/2 "2018-10-12T14:32:13Z")
> [@Tamas\_Papp](#):
>
> gen(g::Gen{T}) where T = T(ntuple(i -\> genfield(g.rng, fieldtype(T, i)), fieldcount(T)))
How about
```julia
gen(g::Gen{T} where T) ::Union{Missing, T} =
T(ntuple(i -> genfield(g.rng, fieldtype(T, i)), fieldcount(T)))
```
---
### Author:  [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [October 14, 2018, 8:54am UTC](https://discourse.julialang.org/t/type-inference-when-generating-a-namedtuple/16222/3 "2018-10-14T08:54:31Z")
Went with `@generated`, as in (the further simplified MWE)
```julia
struct Gen{T <: NamedTuple} end
genfield(::Type{T}) where {T <: Real} = zero(T)
genfield(::Type{Union{Missing, T}}) where T = rand() < 0.5 ? missing : genfield(T)
function gen(g::Gen{T}) where T
if @generated
_names = fieldnames(T)
_types = map(n -> fieldtype(T, n), fieldnames(T))
_vals = Any[:(genfield($fT)) for fT in _types]
:( NamedTuple{$_names, Tuple{$(_types...)}}(($(_vals...),)) )
else
T(ntuple(i -> genfield(fieldtype(T, i)), fieldcount(T)))
end
end
```
then
```julia
julia> g = Gen{NamedTuple{(:a, :b), Tuple{Int, Float64}}}()
Gen{NamedTuple{(:a, :b),Tuple{Int64,Float64}}}()
julia> @code_warntype gen(g)
Body::NamedTuple{(:a, :b),Tuple{Int64,Float64}}
2 1 ─ return (a = 0, b = 0.0) │╻ macro expansion
```
---
### Author:  [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [October 14, 2018, 9:03am UTC](https://discourse.julialang.org/t/type-inference-when-generating-a-namedtuple/16222/4 "2018-10-14T09:03:42Z")
But for
```julia
g = Gen{NamedTuple{(:a, :b), Tuple{Int, Union{Missing, Float64}}}}()
```
I still get
```julia
julia> @code_warntype gen(g)
Body::Any
2 1 ── %1 = Random.GLOBAL_RNG::Random.MersenneTwister │╻╷╷ macro expansion
│ %2 = (Base.getfield)(%1, :idxF)::Int64 ││┃││╷╷╷╷╷╷ genfield
│ %3 = Random.MT_CACHE_F::Int64 │││┃││││││ rand
│ %4 = (%2 === %3)::Bool ││││┃││││││ rand
└─── goto #3 if not %4 │││││┃│││ rand
2 ── %6 = $(Expr(:gc_preserve_begin, :(%1))) ││││││┃│││╷ rand
│ %7 = (Base.getfield)(%1, :state)::Random.DSFMT.DSFMT_state │││││││╻ rand
│ %8 = (Base.getfield)(%1, :vals)::Array{Float64,1} ││││││││┃│││ reserve_1
│ %9 = $(Expr(:foreigncall, :(:jl_array_ptr), Ptr{Float64}, svec(Any), :(:ccall), 1, :(%8)))::Ptr{Float64}
│ %10 = (Base.getfield)(%1, :vals)::Array{Float64,1} ││││││││││╻ macro expansion
│ %11 = (Base.arraylen)(%10)::Int64 │││││││││││╻ length
│ invoke Random.dsfmt_fill_array_close1_open2!(%7::Random.DSFMT.DSFMT_state, %9::Ptr{Float64}, %11::Int64)
│ $(Expr(:gc_preserve_end, :(%6))) │││││││││││
└─── (Base.setfield!)(%1, :idxF, 0) │││││││││││╻╷ mt_setfull!
3 ── goto #4 ││││││││╻ reserve_1
4 ── %16 = (Base.getfield)(%1, :vals)::Array{Float64,1} ││││││││╻╷╷ rand_inbounds
│ %17 = (Base.getfield)(%1, :idxF)::Int64 │││││││││┃│ mt_pop!
│ %18 = (Base.add_int)(%17, 1)::Int64 ││││││││││╻ +
│ (Base.setfield!)(%1, :idxF, %18) ││││││││││╻ setproperty!
│ %20 = (Base.arrayref)(false, %16, %18)::Float64 ││││││││││╻ getindex
└─── goto #5 ││││││││
5 ── goto #6 │││││││
6 ── %23 = (Base.sub_float)(%20, 1.0)::Float64 ││││││╻ -
└─── goto #7 ││││││
7 ── goto #8 │││││
8 ── goto #9 ││││
9 ── %27 = (Base.lt_float)(%23, 0.5)::Bool │││╻ <
└─── goto #11 if not %27 │││
10 ─ %29 = Main.missing::Core.Compiler.Const(missing, false) │││
└─── goto #12 │││
11 ─ goto #12 │││
12 ┄ %32 = φ (#10 => %29, #11 => 0.0)::Union{Missing, Float64} ││
│ %33 = (Core.tuple)(0, %32)::Core.Compiler.PartialTuple(Tuple{Int64,Union{Missing, Float64}}, Any[Const(0, false), Union{Missing, Float64}])
│ %34 = (NamedTuple{(:a, :b),Tuple{Int64,Union{Missing, Float64}}})(%33)::Any
└─── return %34 ││
```
so it did not really help much.