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    <p>There are three options here. The first option is to add</p> <pre><code>{-# LANGUAGE NoMonomorphismRestriction #-} </code></pre> <p>to the top of your module file.</p> <p>The second option is to add an explicit type signature to psqlDriver.</p> <pre><code>psqlDriver :: (Field field, Table table) =&gt; (table -&gt; [field] -&gt; String) -&gt; DatabaseDriver a b </code></pre> <p>The reason for all of this is a little nuanced, and more details can be found <a href="http://www.haskell.org/haskellwiki/Monomorphism_restriction" rel="nofollow">here</a></p> <p>The third option is to change the definition of <code>psqlDriver</code> to</p> <pre><code>psqlDriver = DatabaseDriver insert select </code></pre> <p>However, this is truly ambiguous - there is not reason to prefer any particular instance of <code>Table</code> or <code>Field</code> over another. Perhaps you mean to define <code>DatabaseDriver</code> as follows.</p> <pre><code>data DatabaseDriver table field where DatabaseDriver :: (Table table, Field field) =&gt; { dbInsert :: table -&gt; [field] -&gt; String , dbSelect :: table -&gt; [field] -&gt; String } -&gt; DatabaseDriver table field </code></pre> <p>If the original definition of <code>DatabaseDriver</code> is rewritten as an ADT, it is more obvious why.</p> <p>As it is currently written in the question, the translation to an ADT is</p> <pre><code>{-# LANGUAGE ExistentialQuantification #-} data DatabaseDriver a b = forall table field . (Table table, Field field) =&gt; DatabaseDriver { dbInsert :: table -&gt; [field] -&gt; String , dbSelect :: table -&gt; [field] -&gt; String } </code></pre> <p>Notice the nested <code>forall table field</code>, and how <code>table</code> and <code>field</code> have no relationship to <code>a</code> or <code>b</code>.</p> <p>The intended translation is either</p> <pre><code>data DatabaseDriver table field = (Table table, Field field) =&gt; DatabaseDriver { dbInsert :: table -&gt; [field] -&gt; String , dbSelect :: table -&gt; [field] -&gt; String } </code></pre> <p>or most likely</p> <pre><code>data DatabaseDriver table field = DatabaseDriver { dbInsert :: table -&gt; [field] -&gt; String , dbSelect :: table -&gt; [field] -&gt; String } </code></pre> <p>Having the type class constraints in the definition of <code>DatabaseDriver</code> does not let you remove the type class constraints from any use of <code>DatabaseDriver</code>, in particular <code>psqlDriver</code>. In both of the ADT translations above, the type of <code>psqlDriver</code> is</p> <pre><code>psqlDriver :: (Table table, Field field) =&gt; DatabaseDriver table field </code></pre>
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    1. POAmbiguous type for polymorphic function members of driver GADT
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    1. POAmbiguous type for polymorphic function members of driver GADT
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    1. CONeither of these appear to work for me. The No mono restrictions pragma does clean up the errors some but it still shows an error on the definition and added a signature doesn't clean that up. But I notice that you give a signature of a function while psqlDriver is actually just a record. I suspect records can't have polymorphic functions because that would make them "polymorphic" which doesn't make sense (I guess?) for a data type.
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    2. COAs currently written, `psqlDriver` is not a record, but instead a function. It still requires a second argument, `dbSelect`. Records can definitely be polymorphic. If you want `psqlDriver` to be of type `forall a b . DatabaseDriver a b`, you will need to provide a second argument, possibly named `select` (though this name does not matter, and I only choose this name because of the current name for the first argument `insert`).
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    3. COI now see what was intended and have modified the answer.
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