Support "local" functions in the Editor

[StachuDotNet]

It's often useful to define and then use a function 'inline':

let fn = \val -> ...
let items = List::filter original fn

You can currently define a Block and use it like this, but the editor only allows you to use it in functions that explicitly request a Block such as List::map (at least, as far as I can tell). For example, you can't currently do the following:

let increment = \i -> i + 1
let ten = increment 9

The new version of the language (in the backend) currently supports this, but the Editor does not.

TODOs:

• design/discuss an interaction model
• update Fluid to support this

[StachuDotNet]

In the meantime, one can define an applyBlock fn like this:

and then use it like

let increment = \i -> i + 1
let ten = applyFn increment 9

(at least, that's how I'm personally working around this limitation)

[pbiggar]

The new version of the language (in the backend) currently supports this, but the Editor does not.

I misunderstood the issue, apologies. I thought we were talking about calling higher-order functions with stdlib functions. Anyway, this isn't currently supported in the language. I guess it should probably be supported? I'd prefer folks used real functions in general but it's undeniable that local functions have their place.

So if we want this we'll need a semantics defined as well the interaction model (basically, a definition of what the AST would be, and what the interpreter would do. This is usually pretty simple, though we'll need to account for incompletes, errorrails and derrors).

Note that 9 |> increment should currently work.

[StachuDotNet]

needs to be reassessed for modern day; folded into #5239, will reference this doc when it comes up