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vogievetsky opened this issue Jul 27, 2015 · 3 comments
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Better TypeScript types for inherited methods #564

vogievetsky opened this issue Jul 27, 2015 · 3 comments
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@vogievetsky
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I have started using immutable.js within a TypeScript (1.5.3) project and it is working great save for one small annoyance.

Please excuse me if this has already been asked before or if there is something obvious that I am missing.

import { List } from 'immutable';

function printNiceList(listOfStrings: List<string>): void {
  console.log(listOfStrings.pop().join(', ') + ', and ' + listOfStrings.last());
}

var countries = List(['USA', 'UK', 'Ukraine', 'Uzbekistan']);

// This works nicely:
printNiceList(countries);

// This does not pass the type check:
printNiceList(countries.rest());
// TSC errors with:
// error TS2345 Argument of type 'Iterable<number, string>' is not assignable to parameter of type 'List<string>'.
// > TS2324 Property 'set' is missing in type 'Iterable<number, string>'.

// The type needs to be explicit to get it to work:
printNiceList(<List<string>>countries.rest());

// Same issue with all inherited methods like: slice, splice, skip, concat, e.t.c

Since I work with List<Blah> a lot I end up having to insert <List<Blah>> all over the code which is a little annoying.

Would it be possible to redefine the inherited method types in the .d.ts file so that these methods' return type is List<Blah> vs Iterable<number, string> ?
@leebyron
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rest() returns a Seq, not a List, so you'll need to accept something more generic. I'd recommend:

printNiceList(listOfStrings: IndexedIterable<string>)

@vogievetsky
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Hi @leebyron ,

Thank you for the reply.

You can not change the type signature of printNiceList because it calls pop. You would get an error of error TS2339 Property 'pop' does not exist on type 'IndexedIterable<string>'.

I am not sure what you mean when you say that rest() does not return a list as the return type of it certainly behaves like a list by having the pop method and satisfying List.isList (as one would expect).

var countries = List(['USA', 'UK', 'Ukraine', 'Uzbekistan']);
console.log(List.isList(countries.rest())); // => true

As far as I can see Immutable's logic is solid, it is only the TS type definition that is lacking as it is indication a more generic return type (and that too is only a small cast inconvenience).

@Keats Keats mentioned this issue Aug 3, 2015
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@leebyron
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leebyron commented Mar 8, 2017

Closing this aging issue - latest in master has excellent types which will be released soon

@leebyron leebyron closed this as completed Mar 8, 2017
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@leebyron @vogievetsky