API consistency review

[StefanKarpinski]

I'm starting this as a place to leave notes about things to make sure to consider when checking for API consistency in Julia 1.0.

• Convention prioritization. Listing and prioritizing our what-comes-first conventions in terms of function arguments for do-blocks, IO arguments for functions that print, outputs for in-place functions, etc (official conventional argument precedence #19150).

• Positional vs keyword arguments. Long ago we didn't have keyword arguments. They're still sometimes avoided for performance considerations. We should make this choice based on what makes the best API, not on that kind of historical baggage (keyword performance issues should also be addressed so that this is no longer a consideration).

• Metaprogramming tools. We have a lot of tools like @code_xxx that are paired with underlying functions like code_xxx. These should behave consistently: similar signatures, if there are functions with similar signatures, make sure they have similar macro versions. Ideally, they should all return values, rather than some returning values and others printing results, although that might be hard for things like LLVM code and assembly code.

• IO <=> file name equivalence. We generally allow file names as strings to be passed in place of IO objects and the standard behavior is to open the file in the appropriate mode, pass the resulting IO object to the same function with the same arguments, and then ensure that the IO object is closed afterwards. Verify that all appropriate IO-accepting functions follow this pattern.

• Reducers APIs. Make sure reducers have consistent behaviors – all take a map function before reduction; congruent dimension arguments, etc.

• Dimension arguments. Consistent treatment of "calculate across this [these] dimension[s]" input arguments, what types are allowed etc, consider whether doing these as keyword args might be desired.

• Mutating/non-mutating pairs. Check that non-mutating functions are paired with mutating functions where it makes sense and vice versa.

• Tuple vs. vararg. Check that there is general consistency between whether functions take a tuple as the last argument or a vararg.

• Unions vs. nullables vs. errors. Consistent rules on when functions should throw errors, and when they should return Nullables or Unions (e.g. parse/tryparse, match, etc.).

• Support generators as widely as possible. Make sure any function that could sensibly work with generators does so. We're pretty good about this already, but I'm guessing we've missed a few.

• Output type selection. Be consistent about whether "output type" API's should be in terms of element type or overall container type (ref Functions that return arrays with eltype as input should use container type instead? #11557 and RFC/WIP: Support array type as input for functions returning AbstractArray instance #16740).

• Pick a name. There are a few functions/operators with aliases. I think this is fine in cases where one of the names is non-ASCII and the ASCII version is provided so people can still write pure-ASCII code, but there are also cases like <: which is an alias for issubtype where both names are ASCII. We should pick one and deprecated the other. We deprecated is in favor of === and should do similarly here.

• Consistency with DataStructures. It's somewhat beyond the scope of Base Julia, but we should make sure that all of collections in DataStructures have consistent APIs with those provided by Base. The connection in the other direction is that some of those types may inform how we end up designing the APIs in Base since we want them to extend smoothly and consistently.

• NaNs vs. DomainErrors. See NaN vs wild (or, what's a DomainError, really?) #5234 – have a policy for when to do which and make sure it is followed consistently.

• Collection <=> generator. Sometimes you want a collection, sometimes you want a generator. We should go through all our APIs and make sure there's an option for both where it makes sense. Once upon a time, there was a convention to use an uppercase name for the generator version and a lowercase name for the version that's eager and returns a new collection. But no one ever paid any attention to that, so maybe we need a new convention.

• Higher order functions on associatives. Currently some higher order functions iterate over associative collections with signature (k,v) – e.g. map, filter. Others iterate over pairs, i.e. with signature kv, requiring the body to explicitly destructure the pair into k and v – e.g. all, any. This should be reviewed and made consistent.

• Convert vs. construct. Allow conversion where appropriate. E.g. there have been multiple issues/questions about convert(String, 'x'). In general, conversion is appropriate when there is a single canonical transformation. Conversion of strings into numbers in general isn't appropriate because there are many textual ways to represent numbers, so we need to parse instead, with options. There's a single canonical way to represent version numbers as strings, however, so we may convert those. We should apply this logic carefully and universally.

• Review completeness of collections API. We should look at the standard library functions for collections provided by other languages and make sure we have a way of expressing the common operations they have. For example, we don't have a flatten function or a concat function. We probably should.

• Underscore audit.

[ararslan]

Apologies if this isn't the appropriate place to mention this, but it would be nice to be more consistent with underscores in function names going forward.

[StefanKarpinski]

No, this is a good place for that. And yes, we should strive to eliminate all names where underscores are necessary :)

[tkelman]

• consistent treatment of "calculate across this [these] dimension[s]" input arguments, what types are allowed etc, consider whether doing these as keyword args might be desired
• listing and prioritizing our what-comes-first conventions in terms of function arguments for do-blocks, IO arguments for functions that print, outputs for in-place functions, etc (edit: thought there might already be one open for this)

[ararslan]

For @tkelman's second point, see #19150

[ararslan]

There was also a recent Julep regarding the API for find and related functions: https://github.com/JuliaLang/Juleps/blob/master/Find.md

[shashi]

Should we deprecate put! and take! on channels (and maybe do the same for futures) since we have push! and shift! on them? Just suggesting removing 2 redundant words in the API.

I am suspicious of shift! being user friendly. A candidate is fetch! we already have fetch which is the non-mutating version of take!

ref #13538 #12469

@amitmurthy @malmaud

Edit: It would even make sense to reuse send and recv on channels. (I'm surprised that these are only used for UDPSockets at the moment)

[amitmurthy]

+1 for replacing put!/take! with push!/fetch!

[nalimilan]

I'll add renaming @inferred to @test_inferred.

[martinholters]

Double-check that specializations are consistent with the more generic functions, i.e. not something like #20233.

[dpsanders]

Review all exported functions to check if any can be eliminated by replacing them with multiple dispatch, e.g. print_with_color

[StefanKarpinski]

The typical pairing is push! and shift! when working with a queue-like data structure.

[StefanKarpinski]

If we're not going to use the typical name pairing for this kind of data structure because we're worried that the operation entails communication overhead that isn't adequately conveyed by those names, then I don't think push! makes sense either. send and recv really might be better.

[malmaud]

Maybe double-check that there is general consistency between whether functions take a tuple as the last argument or a vararg.