Functional Typescript

Well typed constructs for functional style programming.

Pipe

Pass values through operations/tasks and guard against errors in a chain-like style without try..catch blocks or variable declarations (let).

function pipe<Current>(data?: Current): Pipe<Current>;

Current is the type of the value being passed at a point in the chain (in the previous case, the start).

interface Pipe<Current> { ... }

Members:

• .guard() Adds a guarding level with the type of possible errors thrown within.

  This method is analogous to announcing that there will be a new try..catch block without actually creating it yet, but only declaring error types that belong in the catch statements.

  For this chained approach, some things call for a different order than the usual.

  Multiple continuous chains for this method is equivalent to calling it once with an union type, while calling it at different points in the chain is analogous to a new nested try..catch block.

  function guard<Error>(): Pipe<Current>;

• .try(task) Executes a task while wrapped in a try..catch block.

  This method is analogous to opening the try block (or continue within if it is already open). task is the (or therefore, part of the) statements enclosed within.

  function try(task: (data: Current) => void): Pipe<Current>;
  function try<Next>(task: (data: Current) => Next): Pipe<Next>;

  If task returns a value, the type Next becomes the new Current. Otherwise, the last value is passed down the chain, maintaining Current.

• .do(task) Executes a task without any error handling.

  Use in place of try if sure the task will not throw. If it throws, the error will be thrown out of the pipe as if there were no catch blocks for it.

  function do(task: (data: Current) => void): Pipe<Current>;
  function do<Next>(task: (data: Current) => Next): Pipe<Next>;

• .catch(handler, level?) Provides a handler for errors caught in any try up to the top or the number of guarded levels when indicated.

  The handler executes only when an accounted (indicated by level) try throws an error. If the handler returns a value, the chain is recovered with that value, otherwise, the chain is halted.

  If there are no catch calls, the pipe may halt at any point silently.

  function catch(handler: (error: Error, lastData?: Previous) => void | Current, level?: number): Pipe<Current>;

  If level is not provided, the handler takes into account all the trys in the chain. Otherwise, it must be a value between 0 and the total number of guarded levels (guards followed by trys)

  Error type is the union type of guards from all accounted levels.

  Previous type is the union type for all the previous Current types throughout the chain from all accounted levels.

Usage:

pipe()
  .guard<'W'>()
  .try(() => { /* may throw 'W', returns 'a' */ })
  .guard<'X'>()
  .try(() => { /* may throw 'X', returns 'b' */ })
  .guard<'Y'>()
  .try(() => { /* may throw 'Y', returns 'c' */ })
  .guard<'Z'>()
  .try(() => { /* may throw 'Z', returns 'd' */ })
  .try(() => { /* may throw 'Z', returns 'e' */ })
  .catch((thrown, saved) => {
  // thrown: 'Z'
  // saved: 'c' | 'd'
  }, 0) // current level
  .catch((thrown, saved) => {
  // thrown: 'X' | 'Y'
  // saved: 'a' | 'b'
  }, 1) // one level above
  .catch((thrown) => {
  // thrown: 'W'
  }); // all levels