replace for non-collection arguments

[tillfalko]

From the documentation:

replace(A, old_new::Pair...; [count::Integer])

Return a copy of collection A where, for each pair old=>new in old_new,
all occurrences of old are replaced by new.

Yet A is not actually constrained to collections. This seems like a wasted opportunity to use Julia's type system to prevent bugs like the following.

x = 2.2
replace(x, NaN=>0)

ERROR: MethodError: no method matching similar(::Float64, ::Type{Float64})

Possible Solutions (Discussion)

1. Somehow restrict the methods of replace to collections. I'm not sure if this is possible as there does not seem to be an AbstractCollection that encompasses AbstractArray, AbstractSet, AbstractString, etc.
2. Support non-collection values of A. While the example above could be done with ifelse or the ternary operator, replace could be very useful for non-iterable values of A. For example I often find myself wanting to write

    (some function)
    return replace(x, Inf=>NaN, -Inf=>NaN)
end

[Moelf]

what's wrong with the current behavior (error?) is the issue that we should error earlier, before hitting similar?

[tillfalko]

sort and in are both 'meant' to be used on collections. Let's see how they behave when called with e.g. floats.

When you call sort(3.3) you get ERROR: MethodError: no method matching sort(::Float64), which makes sense.

When you call 3.3 in 3.3 you get true, which is less intuitive but arguably useful.

I would argue that calling replace(3.3, NaN=>0) should return either 3.3 or ERROR: MethodError: no method matching replace(::Float64, ::Pair{Float64, Int64}).

[LilithHafner]

1. Somehow restrict the methods of replace to collections. I'm not sure if this is possible as there does not seem to be an AbstractCollection that encompasses AbstractArray, AbstractSet, AbstractString, etc.

2. Support non-collection values of A. While the example above could be done with ifelse or the ternary operator, replace could be very useful for non-iterable values of A. For example I often find myself wanting to write

What exactly do you mean by "collection"? Are you referring to iteratebles? Iterables with similar defined? Iterables that can hold more than one element?

(FYI 2.2 is iterable)

[tillfalko]

What exactly do you mean by "collection"?

I used that word because it is used in the docstring to replace.

(FYI 2.2 is iterable)

In that case would it not make sense for replace to support Number the way it does Tuple?

julia/base/set.jl

Line 994 in 7fa26f0

### replace for tuples

EDIT: I renamed this issue to better reflect the topic.

[LilithHafner]

In that case would it not make sense for replace to support Number the way it does Tuple?

Sounds reasonable to me.

[tillfalko]

I would be happy to write up a PR with this if you think this is a good idea.

[LilithHafner]

I can't guarantee we'd merge it; as with all new features, we'd need to be confident it adds more value than the cost of the API expansion. That said, I do think it makes sense to write up a PR and then we can talk about it at triage.

Personally, I'm weakly in favor of adding that method.