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    <p>Main problem is you wish to have permutation instead single value from <code>view</code>. We have only one type which allow permutation - record.</p> <p>So, we can add new data, record type</p> <pre><code>data B = F|T -- just shorter name for Bool in patterns data Palette = P {isW, isU, isB, isR, isG :: B} bool2b :: Bool -&gt; B bool2b True = T bool2b False = F viewColors :: Card -&gt; Palette viewColors (Card colors) = let m = bool2b . (`member` colors) in P {isW = m W, isU = m U, isB = m B, isR = m R, isG = m G} foo :: Card -&gt; String foo (viewColors -&gt; P {isW=T, isB=T}) = "card is white and black" foo _ = "whatever" </code></pre> <hr> <p><strong>UPDATED</strong></p> <p>We also could <strong>deny</strong> wrong patterns. But this solution is more ugly, but it allow use "classic" patterns</p> <pre><code>{-# LANGUAGE GADTs #-} {-# LANGUAGE EmptyDataDecls #-} {-# LANGUAGE RankNTypes #-} data Color = W | U | B | R | G deriving (Eq) data W' data U' data B' data R' data G' data Color' a where W' :: Color' W' U' :: Color' U' B' :: Color' B' R' :: Color' R' G' :: Color' G' data M a = N | J a -- just shorter name for Maybe a in patterns data Palette = Palette (M (Color' W')) (M (Color' U')) (M (Color' B')) (M (Color' R')) (M (Color' G')) </code></pre> <p>and define <code>viewColor</code>:</p> <pre><code>viewColors :: Card -&gt; Palette viewColors (Card colors) = let m :: Color -&gt; Color' a -&gt; M (Color' a) m c e = if c `member` colors then J e else N in P (m W W') (m U U') (m B B') (m R R') (m G G') foo :: Card -&gt; String foo (viewColors -&gt; Palette (J W') N (J B') N N) = "card is white and black" foo _ = "whatever" </code></pre>
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    1. POEnforced pattern order
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    1. CONice solution, but is there a way to make it for a card with colors WUB to not match `foo (viewColors -> P { isW=T })`? Regardless, this is useful to at least query part of a card's colors.
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    2. CO@ThomasEding , add also denying wrong patterns
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