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POIs there a general way to tell the number of parameters of a function in Haskell?
 

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  1. POIs there a general way to tell the number of parameters of a function in Haskell?
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    <p>I can tell the number of parameters of a function with the following code</p> <pre><code>{-#Language MultiParamTypeClasses#-} {-#Language FunctionalDependencies#-} {-#Language UndecidableInstances#-} data Zero data Succ a class Number a instance Number Zero instance (Number a) =&gt; Number (Succ a) class NotFunction a instance NotFunction Int instance NotFunction Float instance NotFunction (IO a) class (Number n) =&gt; FunctionLevel f n | f -&gt; n where functionLevel :: f -&gt; n instance FunctionLevel Int Zero where functionLevel = undefined instance FunctionLevel Float Zero where functionLevel = undefined instance FunctionLevel (IO a) Zero where functionLevel = undefined instance FunctionLevel Double Zero where functionLevel = undefined instance (FunctionLevel f' n) =&gt; FunctionLevel (a-&gt;f') (Succ n) where functionLevel = undefined </code></pre> <p>And we get:</p> <pre><code>*Main&gt; :t functionLevel (undefined::a-&gt;b-&gt;Int) functionLevel (undefined::a-&gt;b-&gt;Int) :: Succ (Succ Zero) *Main&gt; :t functionLevel (undefined::a-&gt;b-&gt;Double) functionLevel (undefined::a-&gt;b-&gt;Double) :: Succ (Succ Zero) *Main&gt; :t functionLevel (undefined::a-&gt;b-&gt;c-&gt;d-&gt;IO a) functionLevel (undefined::a-&gt;b-&gt;c-&gt;d-&gt;IO a) :: Succ (Succ (Succ (Succ Zero))) *Main&gt; :t functionLevel (undefined::a-&gt;b-&gt;c-&gt;d-&gt;Int) functionLevel (undefined::a-&gt;b-&gt;c-&gt;d-&gt;Int) :: Succ (Succ (Succ (Succ Zero))) </code></pre> <p>As you can see, <code>functionLevel</code> performs just as we expected for functions "ended" with some special types. My question is: could we generalize this to tell parameter number of an arbitrary function?</p>
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    1. COYes: `functionLevel _ :: Succ Zero`, since Haskell only has one-argument functions.
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    2. COIf you don't like that definition, then you should explain what "parameter number" means. How many arguments does `id` have, for instance?
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    3. CO@shachaf Pardon, I made a silly mistake. To tell the number of a parameters of a function whose "last result" is polymorphic is certainly nonesense. But to tell that of a function with concrete "last result" is sensible.
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