allow user control over "first word lookup" / generalization of $PATH

[andychu]

#587 made me think: can we look up our internal readlink LAST? (What about realpath too?)

Right now there is a hard-coded POSIX lookup order:

• keywords (although these are syntactically different)
• special builtins (can't be overridden by functions)
• functions
• builtins
• external processes (ls, etc)

And aliases are in there somewhere. I guess you can have aliases for keywords, so they're really first?

I think I floated an idea like

setvar METAPATH = @(special-builtins procs builtins PATH oil-polyfill)

PATH signifies a bunch of lookups.

Or maybe they should be separated without : so you don't have to parse it.

It could be something like:

setvar METAPATH = @(procs @split($PATH, %colon)  ) 

---

This affects Oil Blocks and the Config Dialect, because blocks will have custom "builtins" tacked onto the end. They temporarily change the lookup order.

For example the opts block in #469 will have arg and flag "builtins"

[andychu]

Possible simpler solution:

We could make a special category so that builtin sleep and builtin readlink work, but sleep and readlink are the external binaries.

But I think that's too confusing and subtle...

[andychu]

Idea:

• bin/osh by default doesn't have new builtins
• when shopt --set oil:basic, the "METAPATH" (or whatever mechanism we choose) gets slurp, getline, json, push, etc.
  • although I think repr is useful enough to be there by default ...

[andychu]

Related thread: https://oilshell.zulipchat.com/#narrow/stream/121540-oil-discuss/topic/builtin.2Fkeyword.20clashes.20(Bash.20Builtin.20compatibility.3A.20.60kil.2E.2E.2E

[andychu]

Related: command_not_found hook in bash, which we don't have.

Someone brought that up

[andychu]

#453 for use bin and use dialect is now very related

[andychu]

Related

• use builtin: for "first word lookup" #453
• internal builtin: for reimplementing (some) external commands in the shell #684

Also relates to putting value.Proc in the var namespace