example code for purescript-native ffi code, using a purescript call-back function

strings-codeunits.cpp

// foreign import countPrefix :: (Char -> Boolean) ->  String -> Int
//
exports["countPrefix"] = [](const boxed& f_) -> boxed {
  return [=](const boxed& s_) -> boxed {
    const string& s = unbox<string>(s_);
    size_t i = 0;
    while ( i<s.length() && unbox<bool>(f_(boxed(s.substr(i, 1)))) ) i++;
    return i;
  };
};

// then call this from purescript:
// logShow $ countPrefix (_ /= 'c') "aaaxxxcccxxx"
// should print 6