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copied!<p>If you know in advance the set of properties a molecule might have, you could define a sum type:</p> <pre><code>data Property = Mass Float | CatalogNum Int | Comment String </code></pre> <p>If you want this type to be extensible, you could use Data.Dynamic as another answer suggests. For instance:</p> <pre><code>data Molecule = Molecule { name :: Maybe String, graph :: Gr Atom Bond, property :: [(String,Dynamic)] } deriving (Show) mass :: Molecule -> Maybe Float mass m = case lookup "mass" (property m) of Nothing -> Nothing Just i -> fromDynamic i </code></pre> <p>You could also get rid of the "stringly-typed" <code>(String,a)</code> pairs, say:</p> <pre><code>-- in Molecule: -- property :: [Dynamic] data Mass = Mass Float mass :: Molecule -> Maybe Mass mass m = ... </code></pre> <p>Neither of these attempts gives much type safety over just parsing out of <code>(String,String)</code> pairs since there is no way to enforce the invariant that the user creates well-formed properties (short of wrapping properties in a new type and hiding the constructors in another module, which again breaks extensibility).</p> <p>What you might want are Ocaml-style polymorphic variants. You could look at <a href="http://hackage.haskell.org/packages/archive/vinyl/latest/doc/html/Data-Vinyl-Lens.html" rel="nofollow">Vinyl</a>, which provides type-safe extensible records.</p> <p>As an aside, you might want to get rid of the <code>Maybe</code> wrapper around the list of properties, since the empty list already encodes the case of no properties.</p>

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