# Writing parameter dependent methods for abstract type

**URL:** <https://discourse.julialang.org/t/writing-parameter-dependent-methods-for-abstract-type/136830>\
**Category:** General Usage\
**Tags:** parametric-types\
**Created:** [April 22, 2026, 3:11pm UTC](https://discourse.julialang.org/t/writing-parameter-dependent-methods-for-abstract-type/136830 "2026-04-22T15:11:26Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Tbl](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tbl/32/5984_2.png) [@Tbl](https://discourse.julialang.org/u/Tbl)\
**Post date:** [April 22, 2026, 3:11pm UTC](https://discourse.julialang.org/t/writing-parameter-dependent-methods-for-abstract-type/136830/1 "2026-04-22T15:11:26Z")

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I want to write method for all subtypes of an abstract type which would make use of type parameter. Can you do that? Example, let us say we have concrete type `MyType{T}` and some method which needs `T`, like

```julia
struct MyType{T} end
Base.convert(::Type{MyType{T},::Type{S}) where {S,T} = MyType{promote_type(T,S)}

```

This works, but can you write it so that it works for all subtypes of some `MyAbstractType`? Something like (disfunctional)

```julia
abstract type MyAbstractType{T} end
Base.convert(::Type{C{T}},::Type{S}) where {S,T, C{T}<:MyAbstractType{T}} = C{promote_type(T,S)}

```

This seem to be decidable, but I do not know how to achieve it. Essentially, when I get as parameter type which is of form `A = B{C}` I need to have access to `B` and `C` separately. To get the same functionality I can just repeat definitions for all subtypes I make, but it made me curious.

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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:** [April 22, 2026, 3:48pm UTC](https://discourse.julialang.org/t/writing-parameter-dependent-methods-for-abstract-type/136830/2 "2026-04-22T15:48:06Z")

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`C{T}<:MyAbstractType{T}` asserts the subtype has 1 shared type parameter to specify, which is not true in general. It’s possible for subtypes to have fewer or more type parameters than a supertype:

```julia-auto
struct A <: MyAbstractType{Int} end
struct B{U,V} <: MyAbstractType{V} end

```

`A` doesn’t share `MyAbstractType`’s parameter and isn’t even parametric, so while `T` could be assigned `Int`, there is no reasonable parametric type to assign `C`. Do we introduce a frequent source of `UndefVarError` for static parameters, or do we try to exclude `A` from dispatching to this method without breaking method specificity somehow? `B` has 2 parameters, and even if we match the obvious supertype parameter to the subtype by the name `V` within the definition, do we assign `C` to `B` requiring 2 input parameters or a partially specified `B` subtype to replace `T`? These don’t have satisfying or intuitive answers, especially when we generalize to the supertype having \>1 type parameter. Static parameters represent entire types `::T` or type parameters `::Vector{T}`, not an unknown parametric type `::T{Int}`.

That said, you’re not out of options. With the allowed `::Type{C}` and `C<:MyAbstractType{T}`, a helper function like `_stripparameter(::Type{<:MyType}) = MyType` can map the concrete type `C` to whichever iterated union you prefer. That looks like ridiculous boilerplate, but you’d need nontrivial decisions for `A` or `B`.

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<div class="post-metadata">

**Author:** ![Tbl](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tbl/32/5984_2.png) [@Tbl](https://discourse.julialang.org/u/Tbl)\
**Post date:** [April 25, 2026, 6:31pm UTC](https://discourse.julialang.org/t/writing-parameter-dependent-methods-for-abstract-type/136830/3 "2026-04-25T18:31:51Z")

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Thanks! So essentially I need to make a convention that all subtypes are parametrised in the same way and then add additional helper function.