A utility to match on expressions built by dynamically building a switch statement and evaluating it with eval

expMatch.js

// try in ramda's repl: https://goo.gl/VsRkoY

const expMatch = cases => fallback => matchVal => {
  const sanitize = maybeString =>
    typeof maybeString === 'string' || maybeString instanceof String
      ? `'${maybeString}'`
      : maybeString
  
  // dynamically build a switch statement to be eval'd
  const switchStart = `switch (true) {\n`
  const switchEnd = `default: return ${sanitize(fallback)}\n}`
  const toCaseString = ([exp, val]) => `case (${exp}): return ${sanitize(val)}\n`
  const switchCases = cases(matchVal).map(toCaseString).join('')
  const switchString = `${switchStart} ${switchCases} ${switchEnd}`
  const iife = `(() => { ${switchString} })()`
  
  return eval(iife)
}

// use a function to delay passing in the value we want to match on
// we could skip this step, but doing this makes it more reusable,
// because it allows partially applying arguments
const caseMap = val => [
  [val <= 0, 'Solid'],
  [val > 0 && val < 100, 'Liquid'],
]

const defaultVal = 'Gas'

expMatch(caseMap)(defaultVal)(38) // "Liquid"