Does anyone know why such a "joke proposal" (as stated by the author itself) was chosen for pretty significant projects like pip and cargo? (edit: I tried to say it began as something small and personal)
(Well, I guess there's not that much to win/lose in the area of config languages, but still)
(Also, I think it works pretty well, so this is not to downplay TOML!)
Well, it was only a joke to begin with. I couldn't stand the complexity or ambiguity of YAML and one night I had a few drinks and banged out my thoughts on something better. When people started writing implementations, I realized that TOML might actually have legs and started tightening it up and removing the snarky bits. I guess these projects felt the same pain I did about config files and presto!
I think the moral of the story is: just put your whacky ideas out there and see what happens. You never know when you'll hit a chord.
There's no one clear place to point to about how the decision was made for Cargo, but some nuggets have been left by authors [1]. The gist seems to be that JSON (and YAML, per that comment) was considered a poor format to author and maintain a config file in, and YAML had no lib in Rust and there was no appetite for anyone to write one.
Pip's choice was made for them in the form of PEP 518 [2]. The authors of PEP 518 explain their rationale [2][3], which largely boils down to XML being too wordy and awkward to hand-edit, JSON awkward to hand-edit, various formats used by existing python tooling were underspecified and implementation-dependent, and YAML's python parsers being fairly complex and hard to vendor.
I was one of the supporters of TOML back when this decision was made. I might have been the first one to suggest it, but I don't remember exactly.
Basically, I wanted something that was (1) simple, (2) terse, (3) supported comments, and (4) supported recursive data structures. Requirement (1) eliminated YAML, requirement (2) eliminated XML, requirement (3) eliminated JSON, and requirement (4) eliminated INI. Of the remaining formats, TOML seemed to be the most popular and had the most traction, so I went with that.
This seems reasonable, but I'd like to add that XML, while certainly not designed as a config language, could be made much more terse by using SGML, the markup metalanguage XML is derived from and of which XML is a subset. SGML has additional constructs for short forms such as tag omission/inference and short references (custom Wiki syntax) targetted at authoring, whereas XML only admits canonical angle bracket markup syntax which is what makes XML verbose and cumbersome for edititing by hand.
(Well, I guess there's not that much to win/lose in the area of config languages, but still) (Also, I think it works pretty well, so this is not to downplay TOML!)