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    <p>As Don Stewart said, type classes in GHC are implemented using "dictionaries". That means that the type class <code>Num</code> is represented as a record of functions (I'm gonna skip the <code>Eq</code> and <code>Show</code> constraints here):</p> <pre><code>class Num a where fromInteger :: Integer -&gt; a ... </code></pre> <p>becomes</p> <pre><code>data Num a = Num { fromInteger :: Integer -&gt; a, ... } </code></pre> <hr> <p>When you define an instance, a new record is created with the functions from the instance:</p> <pre><code>instance Num Integer where fromInteger = id ... </code></pre> <p>becomes</p> <pre><code>numDictForInteger = Num { fromInteger = id, ... } </code></pre> <hr> <p>Now, when you use this function in a polymorphic context, the compiler doesn't know which dictionary to use, so it generates an extra parameter for it:</p> <pre><code>foo :: Num a =&gt; a foo = 1 </code></pre> <p>becomes</p> <pre><code>foo :: Num a -&gt; a foo numDict = fromInteger numDict 1 </code></pre> <p>Notice how the constraint <code>Num a =&gt;</code> becomes a parameter <code>Num a -&gt;</code>.</p> <hr> <p>However, when you remove the polymorphism, the compiler knows which dictionary to use statically, so it goes ahead and inlines it instead of generating a parameter:</p> <pre><code>foo :: Integer foo = 1 </code></pre> <p>becomes</p> <pre><code>foo :: Integer foo = fromInteger numDictForInteger 1 </code></pre> <hr> <p>As a footnote, this is why the monomorphism restriction exists. A polymorphic value would not be a CAF since it requires the dictionary argument. This might cause significantly different performance characteristics from what you might expect and therefore you're forced to explicitly state that this behavior is what you wanted.</p>
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    1. COis that correct:1 is an Integer, warp up by function `fromIntegral`, so it equals to `foo numDict = fromInteger numDict (1 :: Integer)`
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