Defining custom functions in sanctuary

index.js

const $ = require('sanctuary-def')

const def = $.create({
  checkTypes: true,
  env: $.env
})

const bangBang =
  def ('bangBang')
      ({})
      ([$.String, $.String])
      (xs => {
        return xs
                 // convert the string to an array of chars
                 .split('')
                 // throw away the char and replace it with '!'
                 .map( _x => '!' )
                 // put the array back together into a string
                 .join('')
      })

const bangBangBad =
  def ('bangBangBad')
      ({})
      ([$.String, $.String])
      (xs => true)