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POEnforced pattern order
 

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    <p>I'm writing a Magic The Gathering (MTG) game engine in Haskell.</p> <p>For those unfamiliar with MTG, it's a card game where cards can have up to 5 colors: White (W), Blue (U), Black (B), Red (R), and Green (G).</p> <pre><code>{-# LANGUAGE ViewPatterns #-} import Data.Set data Color = W | U | B | R | G deriving (Show, Eq, Ord) data Card = Card (Set Color) -- simplified Card type with only its colors viewColors :: Card -&gt; [Color] viewColors (Card colors) = toList colors </code></pre> <hr> <p>What I would like to do is pattern match on colors like so:</p> <pre><code>foo :: Card -&gt; String foo (viewColors -&gt; [W, B]) = "card is white and black" foo _ = "whatever" </code></pre> <p>So far, so good. But there is one problem here: I can type the order of colors incorrectly in the view pattern like so:</p> <pre><code>bar :: Card -&gt; String bar (viewColors -&gt; [B, W]) = "this will never get hit" bar _ = "whatever" </code></pre> <p>Of course, I could have written <code>viewColors</code> in a way that directly resolves this problem. Or I could use guards, but I'd rather not. Here are a couple ways to do so</p> <pre><code>viewColors :: Card -&gt; (Bool, Bool, Bool, Bool, Bool) viewColors (Card colors) = let m = (`member` colors) in (m W, m U, m B, m R, m G) </code></pre> <p>This solution is overly verbose while pattern matching, even if I use a type isomorphic to <code>Bool</code> but with shorter (and/or meaningful) identifiers. Matching a Green card would look like</p> <pre><code>baz :: Card -&gt; String baz (viewColors -&gt; (False, False, False, False, True)) = "it's green" </code></pre> <hr> <pre><code>data ColorView = W | WU | WUB | ... all combos here viewColors :: Card -&gt; ColorView viewColors (Card colors) = extract correct Colorview from colors </code></pre> <p>This solution has combinatorial explosion. Seems extremely bad to implement, but nice to use, especially if I have a <code>colorViewToList :: ColorView -&gt; [Color]</code> to allow programmatic extraction after the pattern match.</p> <hr> <p>I have no idea if the following can be approximated in Haskell, but the following would be ideal:</p> <pre><code>fuz :: Card -&gt; String fuz (viewColors -&gt; (W :* ())) = "it's white" fuz (viewColors -&gt; (W :* U :* ())) = "it's white and blue" fuz (viewColors -&gt; (W :* B :* ())) = "it's white and black" </code></pre> <hr> <p>I'm willing to use advanced language extensions to allow this kind of code: DataKinds, PolyKinds, TypeFamilies, MultiParamTypeClasses, GADTs, you name it.</p> <p>Is something like this possible? Do you have other suggested approaches?</p>
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    1. USThomas Eding
    1. USThomas Eding
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    1. COWhy not use guards? A guard would be almost as pretty as the view pattern: `f card | card `color` [B, W] = ... | card `color` [B,U,W] = ...`. Also, this sounds like a cool project; what are you planning to do with it?
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      1. USTikhon Jelvis
    2. COYou may want to check [this](https://github.com/MedeaMelana/Magic) out.
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      1. USPaul Visschers
    3. CO@TikhonJelvis: I'm not allergic to guards... they are clean and easy. It's just that I love pattern matching a whole lot more. Also, learning the theory is interesting in and of itself.
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