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POHow to get rid of this ambiguity?
 

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  1. POHow to get rid of this ambiguity?
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    <p>I am pretty sure this has been asked before, however I was unable to find the right answer:</p> <p>I tried to eliminate the ambiguity in the following exemplary code snippet:</p> <pre><code>{-# LANGUAGE MultiParamTypeClasses #-} class FooBar a b where foo :: a -&gt; a foo = id bar :: a -&gt; a bar = foo -- ERROR AT THIS LINE </code></pre> <p>I get an error message like so:</p> <pre><code>Ambiguous type variable `b0' in the constraint: (FooBar a b0) arising from a use of `foo' Probable fix: add a type signature that fixes these type variable(s) In the expression: foo In an equation for `bar': bar = foo </code></pre> <p>which is understandable. Note however, that I'm actually unable to follow the compiler's advice and fix the type variable in question: <code>foo</code> doesn't contain <code>b</code> type variable in its type signature. Also this doesn't work:</p> <pre><code> bar = (foo :: FooBar a b =&gt; a -&gt; a) -- ERROR, the same error message </code></pre> <p>Not even with <code>-XScopedTypeVariables</code> enabled.</p> <p>How to tell GHC which FooBar to use? Is it even possible?</p> <p><strong>EDIT</strong></p> <p>Following some answers: yes, I've heard about both functional dependencies and associated types.</p> <p>As to functional dependencies - I am looking for a way to explicitly inform GHC which instance to use (as opposed to asking it to derive the correct type all by itself). </p> <p>As to both - in the real world example that this one comes from, I actually need both of the parameters <code>a</code> and <code>b</code> to be independent.</p> <p>Again: I know this example is extremely silly, I know <code>b</code> is not used in the body of the type class, which makes no sense. This is an extreme simplification of a real world example, that actually does make sense. The real use-case involves "substitutability" of types <code>a</code> using types <code>b</code> and then "unifiability" of types <code>a</code> and <code>b</code> using types <code>c</code> and unfortunately is much longer.</p> <p><strong>EDIT(2)</strong></p> <p>Okay sorry. I think you convinced me after all that I have to refactor the code in order not to have any ill-defined functions (i.e. ones that don't contain all the independent types in their type signatures). Talking to you really helped me think this thing though. Appreciated.</p>
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    1. COCan you explain why would you like to do that? `FooBar`'s implementation only relies on `a`, and does not depend on `b`, so what's the point of mentioning `b`, at all?
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    2. CO@lbolla Yes, I could explain that, however that would take twice as long as all the code snippets here and would involve not one but 3 different type classes. I am afraid you'll have to trust me on that one. It's just an example that could be extended with additional methods that would require both `a` and `b`.
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    3. CO@julkiewicz: I've edited my answer to answer your EDIT.
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