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| <span style='font-size: x-large; font-weight: bold'>Type system extensions in GHC</span> | | <span style='font-size: x-large; font-weight: bold'>Type system extensions in GHC</span> |
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| GHC comes with a rather large collection of type-system extensions (beyond Haskell 98). They are all documented in the [http://www.haskell.org/ghc/docs/latest/html/users_guide/ghc-language-features.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 type-system extensions (beyond Haskell 98). {{GHCUsersGuide|exts||a section on Language Extensions}} But this page is a place to record user-oriented observations, notes, and suggestions on how to use them. |
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| == Type signatures and ambiguity ==
| | * '''[[GHC/Indexed_types|Indexed data types]]''' |
| | | * '''[[GHC/TypeHoles|Agda-like "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 cut-and-paste 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/Stand-alone deriving declarations|"Stand-alone deriving" declarations]]''' |
| <haskell>
| | * '''[[GHC/TypeSigsAndAmbiguity|Type signatures and ambiguity]]''' |
| class C a b where
| | * '''Overlapping instances'''. GHC supports overlapping instances, with carefully specified rules. Make sure you read the [http://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/instances.html#overlapping-instances relevant sections of the user manual]. Here is an application note about [[GHC/AdvancedOverlap|advanced use of overlapping instances]], combined with functional dependencies. |
| foo :: a -> b
| | * '''[[GHC/SuperClass|Super-Class Constraints: Type Families, Type Equalities, FunDep, Overlaps]]''' |
| | | * '''[[GHC/Coercible|Safe coercions using Coercible]]''' |
| konst :: a -> Bool
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| konst x = True
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| f :: (C a b) => a -> Bool
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| f x = konst (foo x)
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| </haskell>
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| If you compile this code, you'll get this error:
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| <pre>
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| Foo1.hs:12:13:
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| Could not deduce (C a b1) from the context (C a b)
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| arising from use of `foo' at Foo1.hs:12:13-17
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| Possible fix: add (C a b1) to the type signature(s) for `f'
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| In the first argument of `konst', namely `(foo x)'
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| In the expression: konst (foo x)
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| In the definition of `f': f x = konst (foo x)
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| </pre>
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| 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>''.
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| 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
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| <haskell>
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| class C a b | a->b where ...
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| </haskell>
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| but you didn't.)
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| 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!
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| 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>.
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| 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:
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| <haskell>
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| instance C Char Bool where ...
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| </haskell>
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| 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.
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| (However, notice that the call <tt>(f 'a')</tt> would be OK if there was an instance like:
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| <haskell>
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| instance C Char w where ...
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| </haskell>
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| Now the constraint <tt>(C Char b)</tt> matches the instance declaration, even though we know nothing about <tt>b</tt>.)
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| 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.
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| == Overlapping instances ==
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| GHC supports overlapping instances, with carefully specified rules. Make sure you read the [http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extensions.html#instance-decls relevant sections of the user manual]. | |
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| Here is an appliation note about [[GHC/AdvancedOverlap|advanced use of overlapping instances]], combined with functional dependencies. | |
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| == Indexed data types and indexed newtypes ==
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| [[GHC/Indexed_types|Indexed 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]. | |
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| == Stand-alone deriving clauses ==
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| Bjorn Bringert has recently implemented [[GHC/Stand-alone deriving declarations|"stand-alone deriving" declarations]].
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| -----------------------
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