How should we render patterns/bindings?

[hmac]

We realised in https://github.com/hackworthltd/vonnegut/pull/285 that we should distinguish between (for want of better terms) patterns and expressions, where patterns define and bind new variables and expressions reference existing variables. In text view, patterns tend to appear on the left hand side of arrows. For example, x is a pattern in each of these:

λ x → ...

case e of
  Just x → ...

let x = e
 in ...

Whereas x is an expression (specifically, a variable) in each of these:

λ y → x

case x of
  ...

let y = x
 in ...

Patterns in Vonnegut have two forms:

• variable patterns, e.g. x
• constructor patterns, e.g. Just x

We currently only allow constructor patterns in case branches and only at the top level, so e.g. Cons (Just x) xs is not a valid pattern.

Patterns have a different a smaller set of rules governing their behaviour compared to expressions. As a user editing a program, the only things you can do to a pattern are:

• rename it
• replace it with another pattern
• (possibly) delete it?

Patterns also don't really "have types" in the same way as expressions, but we can still associate them with the type of expression they match.

(There's probably more info I can add here - I'll do that shortly)

[dhess]

I'm marking this as medium priority, as it's bundled in with all of the other various parameter-ish issues that are already medium priority.

[dhess]

Here's another way in which we should do a better job rendering patterns. Consider the following definition of map :: ∀α . ∀β . (α → β) → List α → List β:

We render the Nil and Cons x xs1 patterns in the case branches without their type applications. For consistency (and I think this will be especially important in eval mode, when the student is visually trying to match patterns in the scrutinee to the corresponding branch pattern), we should render them as Nil @ α and Cons @ α x xs1.

[dhess]

I would like to try rendering constructors in case branches with their Apps in time for user testing. Therefore, I'm going to move this back to "todo" for the Demo project and mark it medium priority.

[dhess]

Nope, not gonna happen in our current frontend.