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    <p>First, your type signature is syntactically wrong (according to my reading of <a href="http://www.haskell.org/onlinereport/haskell2010/haskellch10.html#x17-18000010.5" rel="nofollow">the context-free syntax</a> in the report, although GHC accepts it), it should be</p> <pre><code>foo::(Num a, Num b) =&gt; a -&gt; b -&gt; Bool </code></pre> <p>Regarding the question, you can't do that, <code>(&lt;)</code> has type <code>Ord a =&gt; a -&gt; a -&gt; Bool</code>, so both arguments must have the same type.</p> <p>With a stronger restriction than just <code>Num</code>, you could for example have</p> <pre><code>bar :: (Real a, Real b) =&gt; a -&gt; b -&gt; Bool bar a b = toRational a &lt; toRational b </code></pre> <p>You need to be able to convert both arguments to the same target type (with a meaningful conversion) to compare the values. For types in the <code>Real</code> class, <code>toRational</code> provides such a conversion to a common target type (with the caveat that <code>toRational</code> doesn't work reasonably with NaNs and infinities if the starting type is a floating point type). <code>toInteger</code> would achieve the same for an <code>Integral</code> constraint.</p> <p>There is no such common target type available with just a <code>Num</code> constraint.</p> <p>For your special needs, you may be able to select a target type and define a</p> <pre><code>class Convertible a where toTarget :: a -&gt; Target </code></pre> <p>if <code>Real</code> or <code>Integral</code> are not viable constraints.</p>
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    1. USDaniel Fischer
    1. USDaniel Fischer
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    1. COType signatures like `foo::(Num a) => (Num b) => a -> b -> Bool` are accepted by ghc. Don't know if it's actually against the report.
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      1. USMatvey Aksenov
    2. CONever new that. By my reading of [the syntax](http://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-810004.4.1) ([also here](http://www.haskell.org/onlinereport/haskell2010/haskellch10.html#x17-18000010.5)), the report mandates there being only one `=>`. Probably the GHC parser handles it because it must handle multiple `=>` for higher-rank types.
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