# UndefVarError for Type Variable
**URL:** https://discourse.julialang.org/t/undefvarerror-for-type-variable/77419
**Category:** New to Julia
**Tags:** linearalgebra, error, parametric-types
**Created:** [March 4, 2022, 7:22pm UTC](https://discourse.julialang.org/t/undefvarerror-for-type-variable/77419 "2022-03-04T19:22:01Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@ps-pat](https://discourse.julialang.org/u/ps-pat)
#### Post date: [March 4, 2022, 7:22pm UTC](https://discourse.julialang.org/t/undefvarerror-for-type-variable/77419/1 "2022-03-04T19:22:01Z")
I’m sure I’m missing something obvious, but here we go. I defined methods that dispatch on the type of various “special matrices” from LinearAlgebra. Here is a MWE:
```julia
using LinearAlgebra
for type ∈ [:Symmetric, :LowerTriangular]
@eval f(::Type{$type{T}}) where T = T
end
```
The expected result is returned when called on type `LowerTriangular`:
```julia
julia> f(LowerTriangular{Float64})
Float64
```
However, when called on `Symmetric`, I get:
```julia
julia> f(Symmetric{Float64})
ERROR: UndefVarError: T not defined
Stacktrace:
[1] f(#unused#::Type{Symmetric{Float64, S} where S<:(AbstractMatrix{<:Float64})})
@ Main ./REPL[5]:2
[2] top-level scope
@ REPL[10]:1
```
Interestingly, the following implementation works:
```julia
f(::Type{Symmetric{T, S}}) where {T, S} = T
julia> f(Symmetric{Float64, Matrix{Float64}})
Float64
```
Any idea?
---
### Author:  [@goerch](https://discourse.julialang.org/u/goerch)
#### Post date: [March 4, 2022, 7:56pm UTC](https://discourse.julialang.org/t/undefvarerror-for-type-variable/77419/2 "2022-03-04T19:56:01Z")
Fully specifying the type vars works for me
```julia
using LinearAlgebra
for type ∈ [:Symmetric, :LowerTriangular]
@eval f(::Type{$type{T, A}}) where {T, A} = T
end
@show f(LowerTriangular{Float64,Matrix{Float64}})
@show f(Symmetric{Float64,Matrix{Float64}})
```
yielding
```julia
f(LowerTriangular{Float64, Matrix{Float64}}) = Float64
f(Symmetric{Float64, Matrix{Float64}}) = Float64
```
Don’t know if this helps though.
---
### Author:  [@ps-pat](https://discourse.julialang.org/u/ps-pat)
#### Post date: [March 4, 2022, 8:43pm UTC](https://discourse.julialang.org/t/undefvarerror-for-type-variable/77419/3 "2022-03-04T20:43:33Z")
Yeah that works for me too. I just don’t understand how I can partially specify it for one type and not for the other.
---
### Author:  [@goerch](https://discourse.julialang.org/u/goerch)
#### Post date: [March 4, 2022, 9:08pm UTC](https://discourse.julialang.org/t/undefvarerror-for-type-variable/77419/4 "2022-03-04T21:08:02Z")
I understand my job here as to make things work, sometimes without understanding why. So let us ask another question. What might be the sense of
```julia
@show LowerTriangular{Float32,Matrix{Float64}}([[1, 2], [3, 4], [5, 6]])
```
? Answer:
```julia
ERROR: LoadError: TypeError: in LowerTriangular, in S, expected S<:AbstractMatrix{Float32}, got Type{Matrix{Float64}}
```
So maybe this was designed before the invention of `eltype`?
---
### Author:  [@ps-pat](https://discourse.julialang.org/u/ps-pat)
#### Post date: [March 4, 2022, 9:19pm UTC](https://discourse.julialang.org/t/undefvarerror-for-type-variable/77419/5 "2022-03-04T21:19:22Z")
This is simply due to the definition of `LowerTriangular`: [here](https://github.com/JuliaLang/julia/blob/bf534986350a991e4a1b29126de0342ffd76205e/stdlib/LinearAlgebra/src/triangular.jl#L12-L12)
---
### Author:  [@goerch](https://discourse.julialang.org/u/goerch)
#### Post date: [March 4, 2022, 9:41pm UTC](https://discourse.julialang.org/t/undefvarerror-for-type-variable/77419/6 "2022-03-04T21:41:07Z")
But at least syntactically
```julia
abstract type MyAbstractTriangular{S<:AbstractMatrix} <: AbstractMatrix{eltype(S)} end
```
is accepted.
Edit: this seems to work, too:
```julia
struct MyTriangularMatrix{S<:AbstractMatrix} <: MyAbstractTriangular{S}
A::S
end
function MyTriangularMatrix{T}(A::Matrix{T}) where T
end
tr = MyTriangularMatrix([(1, 2) (3, 4) (5, 6)])
println()
```
---
### Author:  [@goerch](https://discourse.julialang.org/u/goerch)
#### Post date: [March 4, 2022, 9:50pm UTC](https://discourse.julialang.org/t/undefvarerror-for-type-variable/77419/7 "2022-03-04T21:50:19Z")
> [@ps-pat](#):
>
> I just don’t understand how I can partially specify it for one type and not for the other.
This might be an implementation detail? What are you more interested in: this detail or the general idea?