Compile-time checking of string literal arguments based on type

[jbrantly]

Often a method will take as a string the name of a property of some object that it's working on. A common example would be http://backbonejs.org/#Model-get

It would be ideal if TypeScript could provide compile-time checking on these calls to make sure that the string is actually a valid property of the underlying model. As a possible proposal:

interface Person {
    firstName: string
    lastName: string
    phoneNumber: string
}

function get(property: memberof Person) {}

get('firstName') // compiles
get('middleName') // would not compile

In the context of the get function, "memberof Person" would be equivalent to "string".

[danquirk]

Overload on constants already accomplishes this to some degree, although its purpose is mostly about type flow rather than errors on specific strings. The question is do you expect this to be an error:

var fieldNameFromUI = askUserForInput(); // user types 'name'
get(fieldNameFromUI); // is this an error now?

Are you satisfied only getting errors for string literal argument values and never when you pass string typed variables to this function? This is how overload on constant signatures work today. Alternatively they could be changed to not require a general overload that takes a string, but then it would always be an error to pass a variable instead of a string literal which we thought was not very desirable. To do better than that we'd most likely need to implement some sophisticated data flow analysis to try to understand what the literal value in a string variable is (and it would still fail to figure out the value with some frequency).

[jbrantly]

Definitely not looking for any sort of run-time checking. As far as data flow analysis, ex:

var fieldName = 'firstName';
get(fieldName)

I can see that being tricky and personally would not mind if that did not throw any error. I'm not sure if I personally have an opinion on whether or not variables would be allowed at all. I think my leaning would be "yes, variables are allowed". The main issue that I'm trying to solve is that in certain cases you absolutely have to specify a string to access a property and it would be nice if in those cases you could get some compile-time checking so that refactoring/etc is less scary.

[danquirk]

So in that case if we altered the requirements for overload on constant signatures you could get some of this without changing much. Today:

function get(property: 'firstName');
function get(property: 'lastName');
function get(property: 'phoneNumber');
function get(property: string); // this is a required signature today
function get(property: string) {
    // do the getting
}
var result = get('firstName'); // ok
var result2 = get('somethingElse'); // ok
var x = 'hi';
var result3 = get(x); // ok

Alternate reality:

function get(property: 'firstName');
function get(property: 'lastName');
function get(property: 'phoneNumber');
//function get(property: string); // this is no longer required
function get(property: string) {
    // do the getting
}
var result = get('firstName'); // ok
var result2 = get('somethingElse'); // error
var x = 'hi';
var result3 = get(x); // ok

It's a little more verbose to declare than your 'memberof' proposal but it does give you that error checking. It's not clear how often you'd be able to define an API like this that only ever used string literals and never string typed variables though.

[jbrantly]

The problem with the overload on constant signatures is that it requires you to separately define your string constants apart from the actual interface you're working on. A major part of my proposal is that you can use an existing interface to define the valid property names.

In other words, let's say I was writing the TypeScript definition file for Backbone. The definition file today looks something like this:

class Model {
    ...
    get(property: string): any;
    ...
}

It would be much better if I could write this:

class Model<T> {
    ...
    get(property: memberof T): any;
    ...
}

Another example of an API where you would (mostly) use a string literal instead of string typed variables is React: http://facebook.github.io/react/docs/two-way-binding-helpers.html

In particular, the "linkState" function takes an argument which is the name of a property on the state object. Since my state object has an interface it would be ideal if I could ensure at compile-time that the string literal I typed is correct.

[saschanaz]

I would reference old proposals about extending enums:
https://typescript.codeplex.com/discussions/549207
https://typescript.codeplex.com/workitem/1217

[danquirk]

Yeah, I definitely get how overload on constants doesn't scale super well here. Are there many other frameworks where this would be valuable? There's a high bar for adding new syntax so we'd want to ensure it's solving a reasonably large class of issues. We also need to better understand whether it'd be generally useful to be able to define overloads that can only take string literal values and not computed values/variables.

[jbrantly]

Regarding other frameworks, I believe this is a very common pattern. I've spent 30 minutes putting together a list.

Based on a "model"

• Backbone: http://backbonejs.org/#Model-get
• SproutCore: http://guides.sproutcore.com/records.html#-Updating-Records-
• ExtJS: http://docs.sencha.com/extjs/5.0/apidocs/#!/api/Ext.data.Model-method-set
• Ember: http://emberjs.com/guides/models/working-with-records/
• React: http://facebook.github.io/react/docs/two-way-binding-helpers.html
• YUI: https://yuilibrary.com/yui/docs/model/#getting-and-setting-attribute-values
• CanJS: http://canjs.com/docs/can.Model.html (attr() function)
• Kendo UI: http://docs.telerik.com/kendo-ui/api/framework/model#methods-set

Utility mechanisms that could still potentially benefit

• SammyJS: http://sammyjs.org/docs/api/master/all#Sammy.Store
• MooTools: http://mootools.net/docs/core/Element/Element#Element:store
• Underscore: http://underscorejs.org/#pluck http://underscorejs.org/#indexBy
• JQuery (maybe): http://api.jquery.com/jQuery.data/

[danquirk]

Great examples, very useful to guide our thoughts, thanks.

Looking at your earlier examples again, how do you feel about this returning any?

class Model<T> {
    ...
    get(property: memberof T): any;
    ...
}

You'll get nice checking on the input side now but you're going to have to cast on each getter call or else deal with a lot of unsafe code. To do better than that will require some other non-trivial new features which @RyanCavanaugh and I were just talking about and need to be fleshed out a little further.

[jbrantly]

I thought about the return type but left it out as to not make the overall suggestion too big of a bite.

This was my initial thought but has problems. What if there are multiple arguments of type memberof T, which one does membertypeof T refer to?

get(property: memberof T): membertypeof T;
set(property: memberof T, value: membertypeof T);

This solves the "which argument am I referring to" problem, but the membertypeof name seems wrong and not a fan of the operator targeting the property name.

get(property: memberof T): membertypeof property;
set(property: memberof T, value: membertypeof property);

I think this works better.

get(property: memberof T is A): A;
set(property: memberof T is A, value: A)

Unfortunately not sure that I have a great solution although I believe the last suggestion has decent potential.

[saschanaz]

This seems some kind of 'type maps'. Defining this manually will allow potentially more functional string-type enums:

(from my post)

// Has same syntax with interfaces but works only as a type map
typemap Foo {
  madoka: MadokaObject;
  homura: HomuraObject;
  /* ... */
}

class GameCenter {
  // Much shorter function declaration
  createCharacter(charType: Foo is A): A {
  }
}