# Simple compile-time evaluation for struct types

**URL:** <https://discourse.julialang.org/t/simple-compile-time-evaluation-for-struct-types/103326>\
**Category:** New to Julia\
**Created:** [August 29, 2023, 11:45am UTC](https://discourse.julialang.org/t/simple-compile-time-evaluation-for-struct-types/103326 "2023-08-29T11:45:16Z")\
**Posts on this page:** 1\
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**Author:** ![Benny](https://avatars.discourse-cdn.com/v4/letter/b/49beb7/32.png) [@Benny](https://discourse.julialang.org/u/Benny)\
**Post date:** [August 30, 2023, 11:58am UTC](https://discourse.julialang.org/t/simple-compile-time-evaluation-for-struct-types/103326/4 "2023-08-30T11:58:38Z")

</div>

> [@RPS](#):
>
> ` a2::SVector{N1+1, T}`

Thing is, what is `N1+1`? The language doesn’t know the type of `N1`, so it doesn’t know what `+` does. We could constrain `N1` to a concrete type, and in that case there is potential to infer `N1+1`. But a consistent structure needs the `+` method to be pure or never change, which is not guaranteed. Types cannot change structure because there’s no good way to handle existing instances and compiled code. The straightforward rule is requiring every parameter to be provided rather than computed from others.

ComputedFieldTypes doesn’t have this issue because the `@computed struct` header is only a shorthand inner constructor, not an actual type.

```julia
julia> c::Int = -1; function impure_add(a, b) a+b+c end
impure_add (generic function with 1 method)

julia> @computed struct B{N1}
         a::NTuple{impure_add(N1, 1), Int}
       end

julia> fulltype(B{1})
B{1, 1}

julia> c::Int = 0; fulltype(B{1}) # now specifies another type
B{1, 2}

```

Since there isn’t an inner constructor with all the parameters, you must use the shorthand and be careful about what it means at a given moment, or else you get cryptic errors:

```julia
julia> B{1}((1,2))
B{1, 2}((1, 2))

julia> c::Int = 1; B{1}((1,2)) # if you don't adjust properly
ERROR: MethodError: Cannot `convert` an object of type 
  Tuple{Int64{},Int64{}} to an object of type 
  Tuple{Int64{},Int64{},Int64}

```

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