Re: bug-bash Digest, Vol 238, Issue 2

[Martin D Kealey]

Rather than var[i1.i2.i3] I suggest a C-like var[i1][i2][i3] as that avoids
ambiguity for associative arrays whose keys might include ".", and makes it
simpler to add floating point arithmetic later.

I would like to allow space in the syntax to (eventually) distinguish
between an object with a fairly fixed set of fields and a map with an
arbitrary set of keys. Any C string - including the empty string - should
be a valid key, but a field name should have the same structure as a
variable name.

Moreover I'm not so keen on ${var.$key}; I would rather switch the
preferred syntax for associative arrays (maps) to a Perl-like ${var{key}}
so that it's clear from the syntax that arithmetic evaluation should not
occur.

Then we can write ${var[index_expression].field{$key}:-$default}.

Retaining var[key] for associative arrays would be one of the backwards
compatibility options that's only available for old-style (single-level)
lookups.

These might seem like frivolous amendments, but they deeply affect future
features; I can only highly a few things here.

Taken together they enable expression parsing to be a continuation of the
rest of the parser, rather than a separate subsystem that has to be
switched into and out of, and so bash -n will be able to tell you about
syntax errors in your numeric expressions.

There there won't be separate code paths for "parse and evaluate" and
"skip" when handling conditionals; instead there will be just have "parse",
with "evaluate" as a separate (bypassable) step. That improves reliability
and simplifies maintenance. And caching of the parse tree could improve
performance, if that matters.

Backwards compatibility mode would attempt to parse expressions but also
keep the literal text, so that when it later turns out that the variable is
an assoc array, it can use that rather than the expression tree. This would
of course suppress reporting expression syntax errors using bash -n.

-Martin

On Mon, 5 Sep 2022, 05:49 Yair Lenga, <yair.lenga@gmail.com> wrote:

> Putting aside the effort to implement, it might be important to think on
> how the h-data structure will be used by users. For me, the common use case
> will be to implement a simple, small "record" like structure to make it
> easier to write readable code. Bash will never be able to compete with
> Python/Node for large scale jobs, or for performance critical services,
> etc. However, there are many devops/cloud tasks where bash + cloud CLI
> (aws/google/azure) could be a good solution, eliminating the need to build
> "hybrids". In that context, being able to consume, process and produce data
> structures relevant to those tasks can be useful. Clearly, JSON and YAML
> are the most relevant formats.
>
> As a theoretical exercise, looking for feedback for the following, assuming
> that implementation can be done. Suggesting the following:
> * ${var.k1.k2.k3}  -> value           # Should lookup an item via h-data,
> supporting the regular modifiers ('-', for default values, '+' for
> alternate, ...)
> * var[k1.k2.k3]=value                   # Set a specific key, replacing
> sub-documents, if any - e.g. removing any var[.k1.k2.k3.*]
> * var[k1.k2.k3]=(h-value)              # set a specific key to a new
> h-value
> * ${var.k1.k2.k3.*}   -> h->value   # extract h-value string that represent
> the sub-document k1.k2.k3
>
> The 'h-value' representation may be the same format that is currently used
> by the associative array. No need to reinvent here.
>
> Assuming the above are implemented, the missing pieces are "converters" to
> common formats: json, yaml, and possibly XML (yet, there is still a lot of
> those). In theory, following the 'printf' styles:
> * printjson [-v var] h-value
> * readjson var                         # or event var.k1
> * printyaml [-v var] h-value
> * readyaml var                         # or event var.k1
>
> To summarize:
> * Using '.' to identify the hierarchy of the h-data - extension to bash
> syntax.
> * Allow setting a "node" to new value, or new sub-document - may be
> extension
> * Converters to/from standard formats - can be extensions
>
> Looking for feedback
> Yair
>