I&#39;ve recently posted a [question][1] about [syntactic-2.0][2] regarding the definition of `share`. I&#39;ve had this working in **GHC 7.6**:

    {-# LANGUAGE GADTs, TypeOperators, FlexibleContexts #-}
    
    import Data.Syntactic
    import Data.Syntactic.Sugar.BindingT
    
    data Let a where
        Let :: Let (a :-&gt; (a -&gt; b) :-&gt; Full b)
    
    share :: (Let :&lt;: sup,
              sup ~ Domain b, sup ~ Domain a,
              Syntactic a, Syntactic b,
              Syntactic (a -&gt; b),
              SyntacticN (a -&gt; (a -&gt; b) -&gt; b) 
                         fi)
               =&gt; a -&gt; (a -&gt; b) -&gt; b
    share = sugarSym Let

However, GHC 7.8 wants `-XAllowAmbiguousTypes` to compile with that signature. Alternatively, I can replace the `fi` with 

    (ASTF sup (Internal a) -&gt; AST sup ((Internal a) :-&gt; Full (Internal b)) -&gt; ASTF sup (Internal b))

which is the type implied by the fundep on `SyntacticN`. This allows me to avoid the extension. Of course this is 

 - a very long type to add to an already-large signature
 - tiresome to manually derive
 - unnecessary due to the fundep

My questions are:

  1. Is this an acceptable use of `-XAllowAmbiguousTypes`?
  2. In general, when should this extension be used? An answer [here][3] suggests &quot;it is almost never a good idea&quot;.
  3. Though I&#39;ve read [the docs][4], I&#39;m still having trouble deciding if a constraint is ambiguous or not. Specifically, consider this function from Data.Syntactic.Sugar:

        sugarSym :: (sub :&lt;: AST sup, ApplySym sig fi sup, SyntacticN f fi) 
                 =&gt; sub sig -&gt; f
        sugarSym = sugarN . appSym
    
   It appears to me that `fi` (and possibly `sup`) should be ambiguous here, but it compiles without the extension. Why is `sugarSym` unambiguous while `share` is? Since `share` is an application of `sugarSym`, the `share` constraints all come straight from `sugarSym`.


  [1]: https://stackoverflow.com/questions/23448150/techniques-for-tracing-constraints
  [2]: https://github.com/emilaxelsson/syntactic
  [3]: https://stackoverflow.com/questions/23461560/how-can-i-extract-this-polymorphic-recursion-function/23462397#23462397
  [4]: http://www.haskell.org/ghc/docs/7.8.2/html/users_guide/other-type-extensions.html#ambiguity

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When is -XAllowAmbiguousTypes appropriate?