# Generic types in parametric struct constructor

**URL:** <https://discourse.julialang.org/t/generic-types-in-parametric-struct-constructor/31530>\
**Category:** General Usage\
**Created:** [November 26, 2019, 1:49pm UTC](https://discourse.julialang.org/t/generic-types-in-parametric-struct-constructor/31530 "2019-11-26T13:49:45Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![HenriDeh](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/henrideh/32/8316_2.png) [@HenriDeh](https://discourse.julialang.org/u/HenriDeh)\
**Post date:** [November 26, 2019, 1:49pm UTC](https://discourse.julialang.org/t/generic-types-in-parametric-struct-constructor/31530/1 "2019-11-26T13:49:45Z")

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Why does this not work ?

```julia
using Distributions
Uniform{Float32}(0.0,1.0)
ERROR: MethodError: no method matching Uniform{Float32}(::Float64, ::Float64)
Closest candidates are:
  Uniform{Float32}(::T<:Real, ::T<:Real) where T<:Real at /home/dehaybe/.julia/packages/Distributions/tfkz4/src/univariate/continuous/uniform.jl:29
Stacktrace:
 [1] top-level scope at none:0

```

I’m not looking for a fix here but I’d like to know why my Float64 arguments are not considered as subtypes of Real and why the closest candidate is not used.

---

<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:** [November 26, 2019, 1:59pm UTC](https://discourse.julialang.org/t/generic-types-in-parametric-struct-constructor/31530/2 "2019-11-26T13:59:02Z")

</div>

The actual candidate is:

```julia
Uniform{T}(a::T, b::T) where {T <: Real} = new{T}(a, b)

```

from here: [https://github.com/JuliaStats/Distributions.jl/blob/a2304a8811f42a7203c76950ddf5f3a5bceb4533/src/univariate/continuous/uniform.jl#L29](https://github.com/JuliaStats/Distributions.jl/blob/a2304a8811f42a7203c76950ddf5f3a5bceb4533/src/univariate/continuous/uniform.jl#L29) (assuming your `Distributions.jl` is the same as `master`). That signature requires that the type in `{T}` be the same as the types of `a` and `b`, so you would want something like:

```julia
Uniform{Float32}(Float32(0.0), Float32(1.0))

```

It would be easy for Distributions.jl to add an additional inner constructor (or just replace the existing one) to make your original code work as intended:

```julia
Uniform{T}(a, b) where {T} = new{T}(a, b)

```

And I agree that the way Julia prints that method mismatch error is awkward. Is this on the latest version of Julia?

---

<div class="post-metadata">

**Author:** ![HenriDeh](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/henrideh/32/8316_2.png) [@HenriDeh](https://discourse.julialang.org/u/HenriDeh)\
**Post date:** [November 26, 2019, 2:01pm UTC](https://discourse.julialang.org/t/generic-types-in-parametric-struct-constructor/31530/3 "2019-11-26T14:01:46Z")

</div>

Thank you 🙂

Yes it’s Julia 1.2.0.