Why should I not "use unnecessary static parameters"

General Usage
1

The style guide says Don’t use unnecessary static parameters but does not explain why.
Is this just a question of style or are there other considerations?

2

The style guide does explain it:

especially if T is not used in the function body. Even if T is used, it can be replaced with typeof(x) if convenient. There is no performance difference. Note that this is not a general caution against static parameters, just against uses where they are not needed.

3

There is no performance difference, so is this just a caution against typing to much? I did in fact read the style guide :wink: . My question was about an explanation beyond what was in the guide.

4

In a sense, yes. Newcomers to Julia sometimes assume that the redundant T in foo(x::T) where {T} is needed for performance. Generally it isn’t, but note the exceptions.

5

Yes, there is definitely a difference between foo(x) and foo(x::T) where {T}. But to make it clear, is there some subtle difference between foo(x::Real) and foo(x::T) where {T<:Real}?

6

Not from the perspective of Julia’s compiler (again, given the qualifications above, if you don’t otherwise need T etc).

No. And foo(x) is implicitly foo(x::Any), just replace Any with Real and it is the same example, just shorter because Any is assumed.

7

There is (or at least used to be) a difference with respect to no-specialize heuristics for higher-order functions: higher(fun::Function, args...) gives the compiler license to heuristically decide whether to specialize on typeof(fun). If you instead write higher(fun::F, args...) where {F<:Function}, then you force specialization (most of the time the compiler will specialize anyways).

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8

Is this the case? The documentation linked above says that

g_func(g::Function, num) = ntuple(g, div(num, 2))

won’t specialize. Is it a hint rather than a guarantee?

9

More of a hint. If you don’t want to specialize, use @nospecialize :slight_smile:

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10

Yep, if you don’t want to specialize, use @nospecialize, if you absolutely want to specialize, use the foo(x::T) where T idiom, and if you trust that the compiler knows best, just write foo(x).

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