# Typed args in function that does a ccall

**URL:** <https://discourse.julialang.org/t/typed-args-in-function-that-does-a-ccall/48715>\
**Category:** New to Julia\
**Created:** [October 20, 2020, 10:44pm UTC](https://discourse.julialang.org/t/typed-args-in-function-that-does-a-ccall/48715 "2020-10-20T22:44:44Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![jtpettit](https://avatars.discourse-cdn.com/v4/letter/j/ea666f/32.png) [@jtpettit](https://discourse.julialang.org/u/jtpettit)\
**Post date:** [October 20, 2020, 10:44pm UTC](https://discourse.julialang.org/t/typed-args-in-function-that-does-a-ccall/48715/1 "2020-10-20T22:44:44Z")

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If I have a non-overloaded function such as

> loadimage(filename) = @ccall LoadImage(filename::Cstring)::Image

Is there any point in changing it to

> loadimage(filename::AbstractString) = @ccall LoadImage(filename::Cstring)::Image

or should I just let ccall deal with it?

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**Author:** ![rdeits](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rdeits/32/286_2.png) [@rdeits](https://discourse.julialang.org/u/rdeits)\
**Post date:** [October 20, 2020, 11:08pm UTC](https://discourse.julialang.org/t/typed-args-in-function-that-does-a-ccall/48715/2 "2020-10-20T23:08:36Z")

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Changing it to `::AbstractString` would:

- Document the intended kind of input for the function, which could be helpful to future readers or users
- Throw a possibly more helpful `MethodError` if a user passes in something else
- Allow you to also implement other kinds of image loaders, e.g. `loadimage(::IOStream)` and let the compiler pick the appropriate one.

If none of those points are important for your current code, then you don’t need to worry about them. You can keep doing `loadimage(filename)` without any issues.

On the other hand, it would also make your function slightly less general: A user might construct their own type which implements `unsafe_convert(::Type{CString}, ...)` but which is not a subtype of `AbstractString`, in which case they would not be able to use your `loadimage`. However, that’s a pretty odd thing to do, and I would be tempted to blame the user for their weird type definition in that case 🙂

If you’re looking for an opinion, I would personally prefer `loadimage(filename::AbstractString)`, as it’s more explicit about the function’s intended use without being too restrictive of the input type.

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**Author:** ![jtpettit](https://avatars.discourse-cdn.com/v4/letter/j/ea666f/32.png) [@jtpettit](https://discourse.julialang.org/u/jtpettit)\
**Post date:** [October 20, 2020, 11:11pm UTC](https://discourse.julialang.org/t/typed-args-in-function-that-does-a-ccall/48715/3 "2020-10-20T23:11:53Z")

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Thanks for the fast reply! I was leaning towards changing a lot of these functions and you helped me decide in favor of it.