How to use the sanctuary.fromMaybe function in sanctuary

(xs => x => {
      const typeVarName = S.prop ('typevar') (x);
      const newTypeClassName = S.prop ('typeclass') (x);
      const typeVarClasses = S.fromMaybe
        ([])
        (S.get
          (S.is ($.Array ($.String)))
          (typeVarName)
          (xs));
      return S.insert
        (typeVarName)
        (S.append (newTypeClassName) (typeVarClasses))
        (xs);
    })
    ({});

const operate = curry((op, name, line) =>
  S.fromMaybe(line, $do(function* () {
    const braceStart = yield S.indexOf('{', line);
    const braceEnd   = yield S.indexOf('}', line);
    const removeAfterStart = braceEnd - braceStart + 1;
    const insideBraces = slice(braceStart + 1, braceEnd, line);
    const hasSpaceFirst = isSpace(head(insideBraces));

    return yield S.match(/[a-zA-Z0-9_]+/g, insideBraces)
      .chain(sequence(Just))
      .concat(Just([]))
      .map(names =>
        pipe(
          op(__, names),
          uniq,
          sortBy(toLower),
          join(', '),
          fmt1(hasSpaceFirst ? '{ %s }' : '{%s}'),

const capitalize = (str) => concat(toUpper(head(str)), tail(str));
const pascalCase = pipe(camelCase, capitalize);
const isUpper = (c) => toUpper(c) === c;
const isCapitalized = pipe(head, isUpper);
const pkgNameAsVar = ifElse(isCapitalized, pascalCase, camelCase);

const ALIAS  = /:([^!]+)/;
const EXTEND = /!$/;
const parseAlias = pipe(
  S.match(ALIAS), chain(nth(1)));
const parseExtend = pipe(
  S.match(EXTEND), map(T));
const rm = replace(__, '');
const cleanArg = pipe(...map(rm, [ EXTEND, ALIAS ]));
const orEmpty = S.fromMaybe({});

const parsePositionalArg = (arg) => {
  const cleaned = cleanArg(arg);
  return mergeAll([
    orEmpty(map(objOf('alias'),  parseAlias(arg))),
    orEmpty(map(objOf('extend'), parseExtend(arg))),
    { npa: npa(cleaned) }
  ]);
};

const contextForPkg = curry((_require, obj) => {
  const module = _require(obj.name);
  return merge(
    obj.extend ? module : {},
    { [obj.alias]: module }
  );