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copied!<p>I'm not completely sure what examples would count for you, because you mention both non-locality and mutual recursion, and I don't understand whether an example that exhibits just non-locality is enough.</p> <p>I will say that a common technique in Haskell is to write functions whose return type is a class-constrained type variable not mentioned in the argument types. For example, like this:</p> <pre class="lang-hs prettyprint-override"><code>foo :: (Result a) => String -> a foo = toResult . transform -- transform :: String -> String class Result a where toResult :: String -> a -- Example implementation of Result class—with this, callers that -- expect foo to return an Integer will get the length of the result -- of transform. instance Result Integer where toResult = length </code></pre> <p>In this case, the concrete type of the result of any call to <code>foo</code> is determined by type inference at the calling site. I.e., the return type of any call to <code>foo</code> is inferred from information not present in the definition of <code>foo</code>.</p> <p>One practical example of this is Haskell's regular expression libraries. The interface uses this pattern so that, instead of having a bunch of different regexp matching functions that return different types, there is a regexp matching operator that is polymorphic on the return type as shown above, and thus the caller's type inference controls what is returned.</p> <p>So for example, if you do regexp matching in a calling context where the inferred return type is an integer, you get back number of matches. If the calling context expects a boolean, you get True if there were any matches. If the calling context expects a list of strings, you get a list of the substrings that matched the regexp. A bunch of other return type-specific behaviors are defined—and you can define your own for arbitrary return types by implementing your own instances of the library's type class for results.</p>

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