Match From Scratch - The Passerine Codex

Building a `match` expression

A match expression takes a value and a number of functions, and tries to apply the value to each function until one successfully matches and runs. A match expression looks as like this:

l = Some (1, 2, 3)

match l {
    Some (m, x, b) -> m * x + b
    None           -> 0
}

Here's how we can match a match expression:

syntax 'match value { arms... } {
    -- TODO: implement the body
}

This should be read as:

The syntax for a match expression starts with the pseudokeyword match, followed by the value to match against, followed by a block where each item in the block gets collected into the list arms.

What about the body? Well:

If no branches are matched, an error is raised.
If are some branches, we try the first branch in a new fiber and see if it matches.
If the function doesn't raise a match error, we found a match!
If the function does raise a match error, we try again with the remaining branches.

Let's start by implementing this as a function:

-- takes a value to match against
-- and a list of functions, branches
match_function = value branches -> {
    if branches.is_empty {
        error Match "No branches matched in match expression"
    }

    result = try { (head branches) value }

    if result is Result.Ok _ {
        Result.Ok v = result; v
    } else if result is Result.Error Match _ {
        match_function value (tail branches)
    } else if result is Result.Error _ {
        -- a different error occurred, so we re-raise it
        Result.Error e = result; error e
    }
}

I know the use of if to handle tasks that pattern matching excels at hurts a little, but remember, that's why we're building a match expression! Using base constructs to create higher-level affordances with little overhead is a core theme of Passerine development.

Here's how you could use match_function, by the way:

-- Note that we're passing a list of functions
description = match_function Banana ("yellow", "soft") [
    Banana ("brown", "mushy") -> "That's not my banana!",

    Banana ("yellow", flesh)
        | flesh != "soft"
        -> "I mean it's yellow, but not soft",

    Banana (peel, "soft")
        | peel != "yellow"
        -> "I mean it's soft, but not yellow",

    Banana ("yellow", "soft") -> "That's my banana!",

    Banana (color, texture)
        -> "Hmm. I've never seen a { texture } { color } banana before...",
]

This is already orders of magnitude better, but passing a list of functions still feels a bit... hacky. Let's use our match macro definition from earlier to make this more ergonomic:

syntax 'match value { arms... } {
    match_function value arms
}

We've added match expression to Passerine, and they already feel like language features*! Isn't that incredible? Here's the above example we used with match_function adapted to match†:

description = match Banana ("yellow", "soft") {
    Banana ("brown", "mushy") -> "That's not my banana!"

    Banana ("yellow", flesh)
        | flesh != "soft"
        -> "I mean it's yellow, but not soft"

    Banana (peel, "soft")
        | peel != "yellow"
        -> "I mean it's soft, but not yellow"

    Banana ("yellow", "soft") -> "That's my banana!"

    Banana (color, texture)
        -> "Hmm. I've never seen a { texture } { color } banana before..."
}

† Plot twist: we just defined the match expression we've been using throughout this entire overview.

The Passerine Codex

Building a match expression

Building a `match` expression