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 <span style='fontsize: xlarge; fontweight: bold'>Type system extensions in GHC</span>   <span style='fontsize: xlarge; fontweight: bold'>Type system extensions in GHC</span> 
   
−  GHC comes with a rather large collection of typesystem extensions (beyond Haskell 98). They are all documented in the [http://www.haskell.org/ghc/docs/latest/html/users_guide/ghclanguagefeatures.html user manual], but this page is a place to record observations, notes, and suggestions on them.  +  GHC comes with a rather large collection of typesystem extensions (beyond Haskell 98). They are all documented in the [http://www.haskell.org/ghc/docs/latest/html/users_guide/ghclanguagefeatures.html user manual], but this page is a place to record useroriented observations, notes, and suggestions on how to use them. 
   
−  == Type signatures and ambiguity ==
 +  * '''[[GHC/Indexed_typesIndexed data types]]''' 
−   +  * '''[[GHC/TypeHolesAgdalike "holes" in GHC]]''' 
−  It's quite common for people to write a function definition without a type signature, load it into GHCi, use <tt>:t</tt> to see what type it has, and then cutandpaste that type into the source code as a type signature. Usually this works fine, but alas not always. Perhaps this is a deficiency in GHC, but here's one way it can happen:
 +  * '''[[GHC/Standalone deriving declarations"Standalone deriving" declarations]]''' 
−  <haskell>
 +  * '''[[GHC/TypeSigsAndAmbiguityType signatures and ambiguity]]''' 
−  class C a b where
 +  * '''Overlapping instances'''. GHC supports overlapping instances, with carefully specified rules. Make sure you read the [http://www.haskell.org/ghc/docs/latest/html/users_guide/typeclassextensions.html#instancedecls relevant sections of the user manual]. Here is an appliation note about [[GHC/AdvancedOverlapadvanced use of overlapping instances]], combined with functional dependencies. 
−  foo :: a > b
 +  
−   +  
−  konst :: a > Bool
 +  
−  konst x = True
 +  
−   +  
−  f :: (C a b) => a > Bool
 +  
−  f x = konst (foo x)
 +  
−  </haskell>
 +  
−  If you compile this code, you'll get this error:
 +  
−  <pre>
 +  
−  Foo1.hs:12:13:
 +  
−  Could not deduce (C a b1) from the context (C a b)
 +  
−  arising from use of `foo' at Foo1.hs:12:1317
 +  
−  Possible fix: add (C a b1) to the type signature(s) for `f'
 +  
−  In the first argument of `konst', namely `(foo x)'
 +  
−  In the expression: konst (foo x)
 +  
−  In the definition of `f': f x = konst (foo x)
 +  
−  </pre>
 +  
−  What's going on? GHC knows, from the type signature that <tt>x::a</tt>. Then applying <tt>foo</tt> means GHC must pick a return type for <tt>foo</tt>, say <tt>b1</tt>, and generates the type constraint <tt>(C a b1)</tt>. The function <tt>konst</tt> just discards its argument, ''so nothing further is known about <tt>b1</tt>''.
 +  
−   +  
−  Now GHC finished typechecking the right hand side of <tt>f</tt>, so next it checks that the constraints ''needed'' in the RHS, namely <tt>(C a b1)</tt>, can be satisfied from the constraints ''provided'' by the type signature, namely <tt>(C a b)</tt>. Alas there is nothing to tell GHC that <tt>b</tt> and <tt>b1</tt> should be identified together; hence the complaint. (Probably you meant to put a functional dependency in the class declaration, thus
 +  
−  <haskell>
 +  
−  class C a b  a>b where ...
 +  
−  </haskell>
 +  
−  but you didn't.)
 +  
−   +  
−  The surprise is that if you comment out the type signature for <tt>f</tt>, the module will load fine into GHCi! Furthermore <tt>:t</tt> will report a type for <tt>f</tt> that is exactly the same as the type signature that was rejected!
 +  
−   +  
−  Here's what's happening. Without the type signature, GHC picks an arbitrary type for <tt>x</tt>, say <tt>x::a</tt>. Then applying <tt>foo</tt> means GHC must pick a return type for <tt>foo</tt>, say <tt>b</tt>, and generates the type constraint <tt>(C a b)</tt>. The function <tt>konst</tt> just discards its argument, so nothing further is known about <tt>b</tt>. Finally, GHC gathers up all the constraints arising from the right hand side, namely <tt>(C a b)</tt>, and puts them into the inferred type of <tt>f</tt>. So GHC ends up saying that <hask>f :: (C a b) => a > Bool</hask>.
 +  
−   +  
−  This is probably a very stupid type. Suppose you called <tt>f</tt> thus: <tt>(f 'a')</tt>. Then you'd get a constraint <tt>(C Char b)</tt> where nothing is known about <tt>b</tt>. If the instances of <tt>C</tt> constrain both type parameters, you'd be in trouble:
 +  
−  <haskell>
 +  
−  instance C Char Bool where ...
 +  
−  </haskell>
 +  
−  The call gives a <tt>(C Char b)</tt> constraint, with absolutely no way to fix <tt>b</tt> to be <tt>Bool</tt>, or indeed anything else. We're back to very much the same situation as before; it's just that the error is deferred until we call <tt>f</tt>, rather than when we define it.
 +  
−   +  
−  (However, notice that the call <tt>(f 'a')</tt> would be OK if there was an instance like:
 +  
−  <haskell>
 +  
−  instance C Char w where ...
 +  
−  </haskell>
 +  
−  Now the constraint <tt>(C Char b)</tt> matches the instance declaration, even though we know nothing about <tt>b</tt>.)
 +  
−   +  
−  This behaviour isn't ideal. It really only arises in programs that are ambiguous anyway (that is, they could never really work), but it is undoubtedly confusing. But I don't know an easy way to improve it. Yet, anyway.
 +  
−   +  
−  == Overlapping instances ==
 +  
−   +  
−  GHC supports overlapping instances, with carefully specified rules. Make sure you read the [http://www.haskell.org/ghc/docs/latest/html/users_guide/typeclassextensions.html#instancedecls relevant sections of the user manual].  +  
−   +  
−  Here is an appliation note about [[GHC/AdvancedOverlapadvanced use of overlapping instances]], combined with functional dependencies.  +  
−   +  
−  == Indexed data types and indexed newtypes ==
 +  
−   +  
−  [[GHC/Indexed_typesIndexed data types]] (including associated data types) are a very recent addition to GHC's type system extensions that is not yet included in the user manual. To use the extension, you need to obtain a version of GHC from [http://hackage.haskell.org/trac/ghc/wiki/Building/GettingTheSources its source repository].
 +  
−   +  
−  == Standalone deriving clauses ==
 +  
−   +  
−  Bjorn Bringert has recently implemented [[GHC/Standalone deriving declarations"standalone deriving" declarations]].
 +  
−   +  
−  == Type holes ==
 +  
−   +  
−  Simon Peyton Jones, Sean Leather and Thijs Alkemade implemented [[GHC/TypeHolesAgdalike "holes" in GHC]], which will ship in GHC 7.8.1.
 +  
−   +  
−  
 +  