What's the reason for using instantiated types?

[CiaranOMara]

This came to my attention when updating TwoBit.Record. Should lines like the following take a type as the first argument instead of an instantiated type?
https://github.com/BioJulia/BioSequences.jl/blob/16ec1ca0a7b1992ad2a9af76dd7da6824ec59802/src/alphabet.jl#L175

For example, should the above line be like the following, or should the following be additionally provided?

 @inline function encode(::Type{$(A){2}}, nt::$(T)) 

[jakobnissen]

In some sense it's arbitrary - with singleton structs, the type and the instance are the same, functionally. However, the Julia style recommendation recommends using instances, so that's what we do, too.

Edit: For the same reason, the following is in the docstring of Alphabet:

* Except for `eltype` which must follow Base conventions, all functions operating
  on `Alphabet` should operate on instances of the alphabet, not the type.

[TransGirlCodes]

I agree, I think in Base generally instances are used for most trait-like singleton things, with the occasional exception using the type e.g. with eltype, so I figure taking their lead will be the best thing for the most part.

[CiaranOMara]

Fair enough. To disambiguate further, in the style guide, under Write functions with argument ordering similar to Julia Base, there is an explanation for when types are used.

Passing a type typically means that the output will have the given type.

This covers the cases of convert, read, parse, .etc.