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    <p>No, there isn't, since if the compiler could make this determination, that would be equivalent to solving the Halting Problem. In general, the fact that two functions call each other in a "loop" pattern is not enough to conclude that actually calling one of the functions will result in a loop.</p> <p>To use a (contrived) example,</p> <pre><code>collatzOdd :: Int -&gt; Int collatzOdd 1 = 1 collatzOdd n = let n' = 3*n+1 in if n' `mod` 2 == 0 then collatzEven n' else collatzOdd n' collatzEven :: Int -&gt; Int collatzEven n = let n' = n `div` 2 in if n' `mod` 2 == 0 then collatzEven n' else collatzOdd n' collatz :: Int -&gt; Int collatz n = if n `mod` 2 == 0 then collatzEven n else collatzOdd n </code></pre> <p>(This is of course not the most natural way to implement the <a href="http://en.wikipedia.org/wiki/Collatz_conjecture" rel="noreferrer">Collatz conjecture</a>, but it illustrates mutually recursive functions.)</p> <p>Now <code>collatzEven</code> and <code>collatzOdd</code> depend on each other, but the Collatz conjecture states that calling <code>collatz</code> terminates for all positive <code>n</code>. If GHC could determine whether a class that had <code>collatzOdd</code> and <code>collatzEven</code> as default definitions had a complete definition or not, then GHC would be able to solve the Collatz conjecture! (This is of course not a proof of the undecideability of the Halting Problem, but it should illustrate why determining whether a mutually recursive set of functions is well-defined is not at all as trivial as it may seem.)</p> <p>In general, since GHC cannot determine this automatically, documentation for Haskell classes will give the "minimal complete definition" necessary for creating an instance of a class.</p>
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    1. POCan GHC warn if class instance is a loop?
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    1. USAlexander Dunlap
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    1. COIt would be nice if there was some way of specifying the minimal complete definition in such a way that the compiler could check it, though.
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    2. COThat's a good point - perhaps a compiler pragma with the class definition.
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