Yes! The 010 Editor's templating language is a nearly unique (in my experience) hybrid between C-like declarations and imperative flow that's exactly what I want for parsing arbitrary binary files.

The template executes like a script (with conditionals and looping) and a line like "int32 myNum;" is actually just syntactic sugar for "read 4 bytes from the current file pointer and label it myNum." Their example probably does a better job explaining it than I can:

  struct FILE {
      struct HEADER {
   char    type[4];
          int     version;
          ushort  numRecords;
      } header <bgcolor=cLtGray>;
  
      struct RECORD {
          int     employeeId;
          char    name[40];
          float salary;
          if( file.header.version > 1 )
              int      numChildren;
          if( file.header.version > 2 )
              time_t   birthDate;
      } record[ file.header.numRecords ];
  
  } file;

( From: https://www.sweetscape.com/010editor/templates.html )

Yep, same!

010 Editor is one of the few commercial tools that I just have to have a license for - I don't use it to make a living, but I've participated in some CTFs and challenges, and just yesterday I noticed that a WAV file had metadata which I didn't know was a thing, so I just opened it in 010 Editor with the WAV template and I could see how the metadata was stored, and all other fields!

If you’re on the Mac, Synalyze It! is somewhat similar.

https://www.synalysis.net/

I used this for viewing mysql binary logs recently to understand the format. I was upset when I realized the $10 version was severely nerfed. With lack of scripting I couldn't write even a basic grammar to parse out the binlog file. At that point I was too annoyed to shell out another $40 for the pro version.

If you are considering this, get the Pro version. I would recommend to the owner to just make the non-pro version free.

What's the lowest level of granularity? Byte level?

Because I'd love templating feature like that works at the bit level.

IOW: you can specify some a bunch of fields to be, say, 3 bits each, and that when all the bits of a byte are used, it simply spills over to the next byte.

Serious question, is it really worth it for $130?

The next time you want to dwell into a file format, download it and do the 30 day trial - maybe there's already an existing template for the file you're looking for - if not, try making your own - it's super easy! Then you can decide if it's worth it. If such a situation never arises, then no, it's not worth it in your case.

I used it a lot when datamining in World of Warcraft - I would find reverse engineered documentation from previous versions of the game, write it into a 010 editor template struct, run it on the DBC file and then I'd usually get a result where the header and the first few fields would be fine, but the rest would be completely wrong. All I did was add a field in the template before the first obviously wrong field, with different, random sizes, to add random padding until the field started looking correct again.

It's a pretty specific case, but it almost always worked!

Okteta is part of the KDE project, GPL, and has similar templating functionality

it's also my hex editor of choice, has been for years