Ambiguity in the grammar between quoted strings and backward compatible arrays

For example how should software interpret the value in key="a b c"? It could be either a string containing multiple spaces a b c or a backward compatible array of three strings (["a", "b", "c"]).

I think the easiest resolution of this ambiguity is to limit quoted arrays to only integer/real/boolean values, the same way '{' arrays are limited to string values. All the extxyz files I've seen in the wild use arrays in this way, so backward compatibility should be preserved. Do you agree @jameskermode?

I believe a string is the intended behavior. There are no old style arrays of strings, I think. Is this actually contradicted by some documentation or by the reference implementation?

The 1D-array section (https://github.com/libAtoms/extxyz#one-dimensional-array-vector) of the readme says

backward compatible: opens with " or {, one or more of the same primitive types (strings only in {}) separated by whitespace, ends with matching " or }. For backward compatibility, a single element backward compatible array is interpreted as a scalar of the same type.

And "primitive types" allow strings, so that's how I understood this section. The pyleri grammar does not allow string

extxyz/grammar/extxyz_kv_grammar.py

Line 60 in 7ad3758

    
           old_one_d_array = Choice(Sequence('"', Choice(ints_sp, ints, floats_sp, floats, bools_sp, bools), '"'),

so I guess the main thing to update is the documentation!

Although, looking at the code the next line in pyleri grammar says

extxyz/grammar/extxyz_kv_grammar.py

Line 61 in 7ad3758

    
           Sequence('{', Choice(ints_sp, ints, floats_sp, floats, bools_sp, bools, strings_sp, strings), '}'))

while the doc says only string values are allowed inside { arrays, so there is some inconsistency here.

No, the intention the opposite of what I think you're saying - string arrays are only allowed in {} (_not_ "only string values are allowed in {}"), i.e. string arrays are not allowed in "". I agree that this could be more clear, so I'll try to improve the wording.

Please take a look at the revised wording, tell me if it's more clear, and if you agree that it correctly communicates what the pyleri grammar actually implements.

The revised wording is much clearer, thanks!