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    <p>If you just want the solution, look at the end.</p> <p>You nearly answered your own question already. Literals in Haskell are overloaded:</p> <pre><code>Prelude&gt; :t 3 3 :: Num a =&gt; a </code></pre> <p>Since <code>(*)</code> also has a <code>Num</code> constraint</p> <pre><code>Prelude&gt; :t (*) (*) :: Num a =&gt; a -&gt; a -&gt; a </code></pre> <p>this extends to the product:</p> <pre><code>Prelude&gt; :t 3 * 20 3 * 20 :: Num a =&gt; a </code></pre> <p>So, depending on context, this can be specialized to be of type <code>Int</code>, <code>Integer</code>, <code>Float</code>, <code>Double</code>, <code>Rational</code> and more, as needed. In particular, as <code>Fractional</code> is a subclass of <code>Num</code>, it can be used without problems in a division, but then the constraint will become stronger and be for class <code>Fractional</code>:</p> <pre><code>Prelude&gt; :t 3 * 20 / 4 3 * 20 / 4 :: Fractional a =&gt; a </code></pre> <p>The big difference is the identifier <code>c</code> is an <code>Integer</code>. The reason why a simple let-binding in GHCi prompt isn't assigned an overloaded type is the dreaded <a href="http://www.haskell.org/haskellwiki/Monomorphism_restriction" rel="noreferrer">monomorphism restriction</a>. In short: if you define a value that doesn't have any explicit arguments, then it cannot have overloaded type unless you provide an explicit type signature. Numeric types are then defaulted to <code>Integer</code>.</p> <p>Once <code>c</code> is an <code>Integer</code>, the result of the multiplication is <code>Integer</code>, too:</p> <pre><code>Prelude&gt; :t 3 * c 3 * c :: Integer </code></pre> <p>And <code>Integer</code> is not in the <code>Fractional</code> class.</p> <p>There are two solutions to this problem.</p> <ol> <li><p>Make sure your identifiers have overloaded type, too. In this case, it would be as simple as saying</p> <pre><code> Prelude&gt; let c :: Num a =&gt; a; c = 20 Prelude&gt; :t c c :: Num a =&gt; a </code></pre></li> <li><p>Use <code>fromIntegral</code> to cast an integral value to an arbitrary numeric value:</p> <pre><code> Prelude&gt; :t fromIntegral fromIntegral :: (Integral a, Num b) =&gt; a -&gt; b Prelude&gt; let c = 20 Prelude&gt; :t c c :: Integer Prelude&gt; :t fromIntegral c fromIntegral c :: Num b =&gt; b Prelude&gt; 3 * fromIntegral c / 4 15.0 </code></pre></li> </ol>
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