Function peek

To solve some problems you need to know what the next element(s) is (are) going to be, or look back at the previous value(s).

so instead of returning the simple value of the incoming iterator, an object will be returned that looks like this:

 {
   value: 'the current value',
   next: [ 'next value', 'next + 1 value' ],
   previous: [ 'previous value', 'previous - 1 value' ],
 }

Example

// the output of: find src \( -type d -printf "%p/\n" , ! -type d -print \)
const str = `src/
src/interface/
src/interface/itr8ToArray.ts
src/interface/itr8Pushable.ts
src/interface/index.ts
src/peer/
src/peer/observable.ts
src/peer/parseJson.ts
src/types.ts
src/index.ts
src/operators/
src/operators/numeric/
src/operators/numeric/max.ts
src/operators/numeric/min.ts
src/operators/numeric/average.ts
src/operators/numeric/total.ts
src/operators/strings/
src/operators/strings/lineByLine.ts
src/operators/strings/stringToChar.ts
src/operators/general/
src/operators/general/uniq.ts
src/operators/general/reduce.ts
src/operators/general/split.ts
src/operators/boolean/
src/operators/boolean/every.ts
src/operators/boolean/some.ts
src/operators/index.ts`;

const printableTreeView = pipe(
  itr8FromString(str),
  lineByLine(),
  peek(),
  // parse the string into a data structure (peeking to next to fill the isLastInLevel & isVeryLast flags)
  map(({value, next, prev}) => {
    const valueSplit = value.split('/');
    const isDir = value.endsWith('/');
    const pathLength = valueSplit.length - (isDir ? 2 : 1);
    const nextIsDir = next[0]?.endsWith('/');
    const nextPathLength = next[0]?.split('/').length - (nextIsDir ? 2 : 1);
    const isLastInLevel = nextPathLength < pathLength;
    const isVeryLast = next.length === 0;
    return { isDir, name: valueSplit[pathLength], level: pathLength, isLastInLevel, isVeryLast };
  }),
  // turn the data structure into a new string
  map(({ isDir, name, level, isLastInLevel, isVeryLast }) => {
    const padding = pipe(
      itr8FromSingleValue(' │ ')
      repeatEach(level - 1),
      itr8ToString,
    );

    const line = !(isLastInLevel || isVeryLast)
               ? ' ├─'
               : ' └─' // (!isVeryLast ? ' └─' : '─┴─')
    ;

    const filename = `${isDir ? '📁' : '📄'} ${name}`;

    return `${padding}${level > 0 ? line : ''}${filename}`;
  }),
  intersperse('\n'),
  itr8ToString,
)
// =>
// 📁 src
//  ├─📁 interface
//  │  ├─📄 itr8ToArray.ts
//  │  ├─📄 itr8Pushable.ts
//  │  └─📄 index.ts
//  ├─📁 peer
//  │  ├─📄 observable.ts
//  │  └─📄 parseJson.ts
//  ├─📄 types.ts
//  ├─📄 index.ts
//  ├─📁 operators
//  │  ├─📁 numeric
//  │  │  ├─📄 max.ts
//  │  │  ├─📄 min.ts
//  │  │  ├─📄 average.ts
//  │  │  └─📄 total.ts
//  │  ├─📁 strings
//  │  │  ├─📄 lineByLine.ts
//  │  │  └─📄 stringToChar.ts
//  │  ├─📁 general
//  │  │  ├─📄 uniq.ts
//  │  │  ├─📄 reduce.ts
//  │  │  └─📄 split.ts
//  │  ├─📁 boolean
//  │  │  ├─📄 every.ts
//  │  │  └─📄 some.ts
//  │  └─📄 index.ts