showandtell

The goal of show-and-tell is to make it easier to write programs that stop to allow the user the opportunity to inspect some state and modify it. Show-and-tell functions much like debuggers that allow for program state to be inspected and tampered with before resuming program execution.

Stories

Stories manage interactive sessions with users, providing access to a set of commands and updating state. A story is created with an array of commands to make available, and then the start method is invoked to defer to the user.

const showandtell = require('showandtell')

const story = new showandtell.Story([
  showandtell.commands.set,
  showandtell.commands.show
])

let state = {test: 32}

story.start(state)
.then((newState) => {
  console.log('new test value:', parseInt(state.test))
})
.catch(console.error)

Once the user invokes the QUIT command, start will resolve to the new state after all of the modifications have taken place.

Commands

Default Commands

SET <variable> <value>

The SET command allows a value to be assigned in the state. variable is a "path" to the state value to update, like test.values.number, which would update a value a state like {test: {values: {number: <gets updated>}}}. The value is simply the new value to assign. Set does not know the type of the value and will not try to guess it, so the value will be assigned as a string. If the full path does not already exist, it will be created. For example. a state {test: null} updated with the variable test.values.number would be transformed to {test: {values: {number: <value>}}}.

SHOW [variable]

The SHOW command simply logs the current value of a value in the state. variable here is also a "path", just like with SET.

If no argument is passed to SHOW then it will display the entire state available as a JSON structure, so that it is easy to construct the path to the variable one might like to set.

Creating Commands

It is easy to define your own command, which you can supply to a story in order to make it available to the user. A command should be configured with four values.

1. name is the name of the command, in upper case, that the user references
2. help is a string describing the argument format for the command
3. args is an array of objects describing the argument the command expects, in order. Each can contain
   • name: the name that the variable will be given in the args object passed to func.
   • help: a string explaining what the argument is expected to be. Optional.
   • default: a default value to supply to the variable if one is not present. Optional.
   • parser: a function that will convert the value supplied by the user to a desired type. Optional.
4. func is a state transition function

The state transition function, func, should accept three arguments:

1. state is an object containing the state available to the user for inspection/modification
2. args is an object containing values for the variables described in the command's args
3. next is a function that should be invoked with an error (or null) if any, and the new state after the command finishes

For example, a command to square a value at the top-level of the state might look like:

const square = new showandtell.Command({
  name: 'SQUARE',
  help: 'SQUARE <variable>',
  args: [{name: 'variable', help: 'The name of a top-level value to compute the square of'}],
  func: function (state, args, next) {
    if (!state.hasOwnProperty(args.variable)) {
      next(new Error(`State does not have top-level key ${args.variable}`), state)
    } else {
      state[args.variable] *= state[args.variable]
      next(null, state)
    }
  }
})

A note on argument parsing

The argument parser functions in a very simple way. If your command accepts N arguments but only M < N are provided by the user, then only the first M arguments will be assigned their respective values. The remaining N - M values will be assigned their defaults if any are provided, otherwise set to undefined.

Arguments can also be provided as strings, so the value "hello world" will be interpreted as a value for one argument, instead of two arguments containing "hello and world" respectively.