# What are valid type parameters?
**URL:** https://discourse.julialang.org/t/what-are-valid-type-parameters/471
**Category:** General Usage
**Tags:** question
**Created:** [November 21, 2016, 2:33pm UTC](https://discourse.julialang.org/t/what-are-valid-type-parameters/471 "2016-11-21T14:33:59Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [November 21, 2016, 2:33pm UTC](https://discourse.julialang.org/t/what-are-valid-type-parameters/471/1 "2016-11-21T14:33:59Z")
I am trying to stuff quite a bit of information into parameters of a type, in the hope that dispatching on this will create efficient code. Doing this, I ran into the question of what is allowed, exactly.
Is the following correct: a value `V` is valid for a type parameter iff either
1. `isbits(V)`
2. `isa(V, Symbol)`
3. `isa(V, Type)`
Is there anything else?
To make things concrete, what I am trying to encode as type parameters are a return type, and a `Vector{Pair{Int,Type}}`. The latter does not work, but I can split the pairs, ending up two vectors, which works.
---
### Author:  [@yuyichao](https://discourse.julialang.org/u/yuyichao)
#### Post date: [November 21, 2016, 2:52pm UTC](https://discourse.julialang.org/t/what-are-valid-type-parameters/471/2 "2016-11-21T14:52:16Z")
Also `Tuple` of `Symbol` and `isbits` type.
Encode return type in parameter is fine (like `convert`) but you shouldn’t try to “help” inference this way.
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### Author:  [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [November 21, 2016, 3:00pm UTC](https://discourse.julialang.org/t/what-are-valid-type-parameters/471/3 "2016-11-21T15:00:04Z")
Thanks. I am not “helping” type inference, I am encoding all the information in type parameters (it does not exist anywhere else). I got the idea from a code example which converts parser spec strings to symbols, and puts them in a type parameter (can’t find the link now).
Another question I ran into is the following: given a `Tuple{...}`, how do I extract the type parameters? Eg from `Tuple{Int,Float64}`, how would I get the `[Int,Float64]`?
EDIT:
```julia
xx(T::Type) = map(name->fieldtype(T,name), fieldnames(T))
```
works. But I have a feeling I probably should not be doing this 😃
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### Author:  [@akis](https://discourse.julialang.org/u/akis)
#### Post date: [November 21, 2016, 3:11pm UTC](https://discourse.julialang.org/t/what-are-valid-type-parameters/471/4 "2016-11-21T15:11:20Z")
> [@Tamas\_Papp](#):
>
> Another question I ran into is the following: given a Tuple{…}, how do I extract the type parameters? Eg from Tuple{Int,Float64}, how would I get the [Int,Float64]?
```julia
julia> [Tuple{Int,Float64}.parameters...]
2-element Array{DataType,1}:
Int64
Float64
```
---
### Author:  [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [November 21, 2016, 3:17pm UTC](https://discourse.julialang.org/t/what-are-valid-type-parameters/471/5 "2016-11-21T15:17:10Z")
> [@akis](#):
>
> ```julia
> [Tuple{Int,Float64}.parameters...]
>
> ```
neat. another question: how could I have discovered this? I could not find it in the manual, and `dump` does not give anything meaningful for `Tuple`s.
---
### Author:  [@akis](https://discourse.julialang.org/u/akis)
#### Post date: [November 21, 2016, 3:20pm UTC](https://discourse.julialang.org/t/what-are-valid-type-parameters/471/6 "2016-11-21T15:20:24Z")
> [@Tamas\_Papp](#):
>
> how could I have discovered this?
```julia
julia> typeof(Tuple{Int,Float64})
DataType
julia> fieldnames(DataType)
17-element Array{Symbol,1}:
[...]
:parameters
[...]
```