parser11.js

char('a')
.run('a')
.fold(
  v => console.log('success', v)
  e => console.log('error', e)
)
// => success a

char('a')
.run('b')
.fold(
  v => console.log('success', v)
  e => console.log('error', e)
)
// => error predicate did not match