# Constrained type parameters
**URL:** https://discourse.julialang.org/t/constrained-type-parameters/50263
**Category:** New to Julia
**Created:** [November 16, 2020, 11:44pm UTC](https://discourse.julialang.org/t/constrained-type-parameters/50263 "2020-11-16T23:44:55Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@eckofoid](https://discourse.julialang.org/u/eckofoid)
#### Post date: [November 16, 2020, 11:44pm UTC](https://discourse.julialang.org/t/constrained-type-parameters/50263/1 "2020-11-16T23:44:55Z")
When defining a type, is it possible to place a constraint on parameter values? For example, consider the following simple type definition:
```julia
struct Orf
start::Int32
stop::Int32
pol::Int8
end
```
Is there any way to add a hint such as `"stop > start"` to this definition? In pseudo-code, it might resemble
```julia
struct Orf
start::Int32
stop::Int32, start > start
pol::Int8
end
```
In this way, data errors could be caught without explicit tests in code.
---
### Author:  [@lmiq](https://discourse.julialang.org/u/lmiq)
#### Post date: [November 16, 2020, 11:52pm UTC](https://discourse.julialang.org/t/constrained-type-parameters/50263/2 "2020-11-16T23:52:12Z")
Check this, the first example is exactly what you need:
[https://docs.julialang.org/en/v1/manual/constructors/#man-inner-constructor-methods](https://docs.julialang.org/en/v1/manual/constructors/#man-inner-constructor-methods)
---
### Author:  [@eckofoid](https://discourse.julialang.org/u/eckofoid)
#### Post date: [November 17, 2020, 8:12pm UTC](https://discourse.julialang.org/t/constrained-type-parameters/50263/3 "2020-11-17T20:12:18Z")
Perfect! Thanks very much for pointing this out.
---
### Author:  [@FedericoStra](https://discourse.julialang.org/u/FedericoStra)
#### Post date: [November 23, 2020, 11:30pm UTC](https://discourse.julialang.org/t/constrained-type-parameters/50263/4 "2020-11-23T23:30:10Z")
A pattern that I like is
```julia
struct OrderedPair
x::Real
y::Real
function OrderedPair(x, y, ::Val{Checks}=Val(true)) where {Checks}
if Checks
x > y || error("out of order")
end
new(x, y)
end
end
```
This way you can avoid the checks when you are 100% sure that the values you are feeding to the constructor satisfy all the requirements. If you call `OrderedPair(x, y, Val(false))`, the checks are statically removed.
---
### Author:  [@mocalvao](https://discourse.julialang.org/u/mocalvao)
#### Post date: [December 18, 2020, 8:08pm UTC](https://discourse.julialang.org/t/constrained-type-parameters/50263/5 "2020-12-18T20:08:42Z")
This seems to be a perfect use case for the macro @assert from the package Parameters.jl, such as:
```julia
using Parameters
struct OrderedPair
x::Real
y::Real; @assert x <= y
end
```
---
### Author:  [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [December 19, 2020, 7:13am UTC](https://discourse.julialang.org/t/constrained-type-parameters/50263/6 "2020-12-19T07:13:54Z")
Note that `@assert` is in `Base`. Perhaps you meant `Parameters.@with_kw`.
---
### Author:  [@mocalvao](https://discourse.julialang.org/u/mocalvao)
#### Post date: [December 19, 2020, 6:36pm UTC](https://discourse.julialang.org/t/constrained-type-parameters/50263/7 "2020-12-19T18:36:21Z")
Thanks for the correction, Tamas\_Papp: I really meant what you said and wanted to call attention to the very useful (at least to me, as a newbie) Parameters.jl package.