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Oils Pure Mode: For Config Files and Functions

The interpreter can run in pure mode

Why does Oils have Pure Mode?

• config files should be isolated to git
• data versus code

• JSON is data.
  • TSV, HTML is data
  • J8 Notation is data.
• Hay is programmable data.
  • it doesn't format your hard drive.

WebAssembly Analogy

WebAssembly is also pure computation, and can be used in a variety of contexts.

It has io too.

When is it active?

--eval-pure

vs --eval

TODO: link to doc/reef

eval() and evalExpr()

vs. io->eval() and io->evalExpr()

TODO: link to doc/reef

Pure Functions

• TODO: func

What's allowed in Pure Mode? Not allowed?

What's not allowed:

• external commands
• any process construct
  • pipelines
  • subshell
  • command sub and process sub
  • background jobs with &
• many builtins
  • most of these do I/O
• globbing - touches the file system
• time and randomness
  • $SECONDS and $RANDOM

• shell-flags for --eval-pure
• func/eval and func/evalExpr
• func - functions are pure
• io and vm - impure behavior is attached to these objects

Notes

Pure mode is a bit like functional programming, but the style inside a function is not functional.

In YSH, it's natural to write pure functions in an imperative style. Just like you can do this in Python or JavaScript (although there is no enforcement, unlike Oils)