Allow user to specify a conversion function for a column

[simonw]

Super advanced: ability to reformat data to populate columns correctly, for example reformatting dates to match. This is likely to be a separate feature, there's a lot of depth to this one and it affects how imports work too.

Might also need a bit of persistence around this in order to remember what those conversions were for future loads into the same table.

_Originally posted by @simonw in #3

[simonw]

Worked with Claude Opus to figure out this recipe for running code hopefully-safely in a worker:

workerScript = `
self.onmessage = function(event) {
  const userCode = event.data.userCode;;
  try {
    const result = eval(userCode);
    self.postMessage({ success: true, result: result });
  } catch (error) {
    self.postMessage({ success: false, error: error.message });
  }
};
`;

function safeEval(userProvidedCode, timeLimit) {
  timeLimit = timeLimit || 1000;
  let worker = new Worker(URL.createObjectURL(new Blob([workerScript], { type: 'text/javascript' })));
  worker.onmessage = function(event) {
    if (event.data.success) {
      console.log('Result:', event.data.result);
    } else {
      console.error('Error:', event.data.error);
    }
    worker.terminate();
    worker = null;
  };
  worker.postMessage({ userCode: userProvidedCode });
  setTimeout(function() {
    if (worker) {
      worker.terminate();
      console.error('Execution timed out');
    }
  }, timeLimit);
}

Claude conversation here: https://gist.github.com/simonw/ef14e52da14e995f986221865ab72a55

[simonw]

... or I could use QuickJS as a sandbox, finally figured out a good recipe for loading that here: https://til.simonwillison.net/npm/self-hosted-quickjs