safe-tidal-cli

This is for running "tidal-as-a-service" (for collaborative editors) in a somewhat safer way.

See tidalcycles/Tidal#627

This package contains a library (safe-tidal) and an executable (safe-tidal-cli).

The indented use case is with https://github.com/munshkr/flok :

node packages/repl/bin/flok-repl.js -H ws://localhost:3000 -s <token> -n tidal -- safe-tidal-cli

Install

cabal install tidal safe-tidal --lib # installs libraries
cabal install safe-tidal  --constraint 'tidal==1.4.9' # installs executable

beware of cabal env-file hell, cf. #5

Goals

The executable safe-tidal-cli aims to provide

• the essential subset of the functionality of a ghci session with BootTidal loaded,
• while being extra resilient to improper input.

safe-tidal-cli will connect to SuperDirt on startup.

Then it accepts text blocks on stdin
where blocks are separated by empty lines. In the intended use case, a block will be sent by the flok server each time a client hits Ctrl-Enter.

Each block contains one expression, e.g., d1 $ s "bd", which safe-tidal-cli will evaluate (by the embedded ghci). The resulting value is an action that gets executed (by the embedded tidal). The execution involves communication with the SuperDirt/SuperCollider back-end.

These expressions appear to be of type IO () but actually we have re-defined d1 etc. to use a safe wrapper that only allows Tidal's top-level operators (cf. #8) and probibits any other form of IO.

You can write any expression of that type, e.g.,

d1 $ s "bd"

let x = .. in do { d1 $ ... ; d2 $ ... }

hush

safe-tidal-cli will reject any expression of any other type, in particular, of type IO a, so no-one can execute readFile "/etc/passwd". It will also reject harmless expressions like 1+2. We may want to allow that.

safe-tidal-cli rejects all non-expressions. That means you cannot interact with the state of the ghci session. You cannot write

• imports (import System.IO.Unsafe),
• definitions (let foo = bar),
• queries (:type, :info, :doc).

We may lift the latter restriction.

safe-tidal-cli will exit when stdin is closed and all blocks are processed.

Related Work

The idea of using the GHC API (via hint) for tidal is not new, cf. https://github.com/tidalcycles/tidali/blob/master/src/Main.hs The idea of wrapping IO to make it safe, is also not new (TODO: references?). Perhaps the combination is new.