[Haskell-cafe] using Haskell more like scripting language

[Dennis Raddle]

Thanks Vanessa. The issue is that not everything will have a position or
velocity. And some things will have more than those. There's a large
overlapping set of possibilities that I can't quite predict until after
I've finished several versions.

However, I think I have a solution. This is actually in Purescript, which
has has extensible record types. Each object will be represented by its own
unique record, but there are mechanisms to write polymorphic code that
operates on just portions of the records. I think that will get me through
the first few versions of this program.

Purescript also has dynamic types, but I suspect those will be hard to
debug. When I'm developing code, I like to get helpful error messages.
Please, no untraceable crashes from many levels deep in the stack. I don't
know if that's what happens when a dynamic type fails to be the expected
type, but I do know that these kinds of crashes are even harder to debug in
Purescript than Haskell, as they happen in the compiled Javascript (which I
don't understand much).

D

Mike


On Sat, Jul 28, 2018 at 9:14 PM, Vanessa McHale <vanessa.mchale at iohk.io>
wrote:

> If everything you use will have a color, position, and velocity I'd just
> use a product type, i.e.
>
> data itemState = ItemState Color Point Vector
>
> or even
>
> data ItemState = ItemState (Maybe Color) (Maybe Point) (Maybe Vector)
>
> Alternately, if you do want to go the dynamic route, `Data.Dynamic` in
> `base` actually has some facilities for dynamic programming
>
> On 07/28/2018 05:04 PM, Dennis Raddle wrote:
>
> I'm actually working in Purescript, but the issue is pretty much the same
> as one would encounter in Haskell.
>
> My application (animation of math-related ideas) is changing constantly.
> It's research, and I don't know where I'll be with it next week, or even
> tomorrow.
>
> For instance, the kinds of "objects" I'm animating (numbers, equations)
> and the way they move and interact with each other is a research project,
> and I'm constantly ripping out parts of my application and scavenging them
> for variations on a theme.
>
> My objects need state. At this point I'm thinking of just putting the
> state into a map with strings as keys and some kind of sum type to
> represent each data item I might need.
>
> So if I want to represent that an object has a particular color and
> position and velocity,
>
> itemState = M.fromList
>   [("color"   , DataColor  blue       )
>   ,("position", DataPoint  500.0 100.0)
>   ,("velocity", DataVector 50.0 45.0  ) ]
>
> Which starts to look more like a dynamically-typed scripting language. But
> it's not hard to work with. I can uses lenses/prisms to inspect and update
> state, and it's okay if my program crashes on encountering malformed state.
>
> What this lets me do is create new objects that mix and match parts of old
> objects, and copy and paste a lot of the code.
>
> Is there a better way to do this? Is there a compromise I can make that
> will allow the compiler to help me out more with finding errors at compile
> time?
>
> D
>
>
>
>
>
>
>
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