How to put a contracts on a hashmap's keys and values

[TobiaszCudnik]

How to put a contracts on a hashmap's keys and values, for example custom key contract and object values.

Right now it's achivable via a custom function contracts which:

• assets it's an object
• checks all keys via object.keys()
• checks all values in a loop using a custom object contract

Is there/will be a syntax for this? How about contract parameters? Maybe

foo :: HashMap TMyKey, TMyObjValue

Or

foo :: { @TMyKey, TMyObjValue }

[disnet]

Don't have syntax for this right now. Closest I think is something like this:

class HashMap
  # non-implementation of a HashMap :)
  get: (key) -> @[key]
  put: (key, val) -> @[key] = val


HashMapC = (keyContract, valContract) ->
  ?{
    get: ((keyContract) -> valContract)
    put: (keyContract, valContract) -> Any
  }

StrHashMap = HashMapC Str, Str

f :: (StrHashMap) -> Any
f = (map) ->
  map.get 4

m = new HashMap

m.put 4, "the number four"
f m # contract violation...

Does this match with what you've been trying?

We definitely need some nicer syntax for stuff like this. I'm leaning towards the parameterisation approach (foo :: MyContract T, V) over a specific new syntax just for HashMaps.

[gregwebs]

I think the problem I was just going to open up a ticket for is somewhat related. I am extending backbone views and models. The preferred way of doing this is

View = Backbone.View.extend
    method: (args) ->

So all the methods are defined in a JSON style. It isn't obvious to me how to add a contract. I can define the method externally, but that leads to some duplication. Perhaps I should add it on after extending?

[disnet]

I think you should be able to do something like:

View :: { 
  method: (C) -> C 
}
View = Backbone.View.extend
  method: (args) -> ...

[gregwebs]

yeah, I know I should be able to do that but then my type definition would be far away from the function location. Backbone views can easily take up 100 lines of code.

[disnet]

Ah, I see. Yeah, there's probably not a great way to do it at the moment.

I'd like to add the ability to define inline contracts with object literals. So one could write:

obj = 
  (Str) -> Str
  meth1: (x) -> ...

  (Num) -> Num
  meth2: (x) -> ...

  Num
  x: 42

This is related to adding support for classes since the grammar here is pretty similar. Hopefully I'll have some time soon to crank this out :)

But this still wouldn't entirely solve your backbone example. If we added inline contracts or tried this today:

spec :: { method : (C) -> C }
spec = { method: (args) -> ... }

View = Backbone.View.extend spec

so the object we pass View.extend is wrapped in a contract. Depending on exactly how extend works you might not get what you expect because there's a conceptual mismatch going on. We really want to put a contract on View not the bit of data (spec) that is used to generate View.

Not sure the best way to address this.

[gregwebs]

method :: (C) -> C
method = (args) -> ...

# create a new copy and manually add methods
View = Backbone.View.extend {}
mkView = () ->
  v = new View
  v.method = method

# call to the outside function
View = Backbone.View.extend
  method : (args...) -> method(args...)

These seem like the only approach where I get a contract next to my function and place the contract on the correct object. Second seems preferable.

[gregwebs]

This seems to work well. A small amount of manageable boilerplate. I need the context, though, so it looks like:

View = Backbone.View.extend
  method : (args...) -> method(this, args...)

I could apply the context (and maybe I will in the future), but I actually kind of like passing it around.

[gregwebs]

The style I am using now is

addContractMethods = (singleton) ->
  singleton.method :: (Str) -> Any 
  singleton.method = (str) ->

mkClass contractMethods