types.js

const $ = require('sanctuary-def');

const Ratio =
  $.NullaryType('package-name/Ratio',
                'https://package.url.io/Ratio',
                x => typeof x === 'number' && (x >= 0 && x <= 1));

//

const def = $.create({
  env: $.env.concat([Ratio]),
  checkTypes: true
});

//

const getPct =
  def('getPct', {},
      [Ratio, $.String],
      x => `${x * 100}%');

types.test.md

> getPct(0.5)
'50%'

> getPct(100)
TypeError: Invalid value

getPct :: Ratio -> String
          ^^^^^
            1

1)  100 :: Number

The value at position 1 is not a member of ‘Ratio’.

See https://package.url.io/Ratio for information about the package-name/Ratio type.