Dhall-lang: List comprehensions

[bch29]

List comprehensions (a la Haskell or Python) are much easier for humans to read than chains of List/map, List/concatMap and List/filter. This is especially true in dhall where you have to provide the type arguments to list functions. For example, I frequently have a need to generate a flat list of unstructured data from multiple lists of structured data, like the below example.

let Shape = < square | circle | triangle >
let Color = < red | green | blue >

let shape_name = \(shape : Shape) -> merge { square = "square", circle = "circle", triangle = "triangle" } shape

let color_name = \(color : Color) -> merge { red = "red", green = "green", blue = "blue" } Color

-- It is much more obvious at a glance what this is doing than the version using List/concatMap then List/map!
let shape_descriptions = 
    [ "${color_name color} ${shape_name shape}"
    | color <- [Color.red, Color.green, Color.blue]
    | shape <- [Shape.square, Shape.circle, Shape.triangle]
    ]

[Gabriel439]

@bch29: Note that even if Dhall were to have language support for do notation or list comprehension notation, there would likely still need to be a type annotation on the bound value, like this:

    [ "${color_name color} ${shape_name shape}"
    | color : Color <- [Color.red, Color.green, Color.blue]
    | shape : Shape <- [Shape.square, Shape.circle, Shape.triangle]
    ]

Would that still be usable for your use case?

[bch29]

Thanks for getting back to me. Yes, this would still be much better than concatMap and map.

[Gabriel439]

This discussion do notation is also related, since list comprehension is a special case of do notation: https://discourse.dhall-lang.org/t/proposal-do-notation-syntax/99/7

[singpolyma]

List comprehensions (a la Haskell or Python) are much easier for humans to read than chains of List/map, List/concatMap and List/filter

I think this is a matter of personal preference. I, for example, have always found list comprehensions to be unreadable.