GHC/Type system
From HaskellWiki
(→Type signatures and ambiguity) |
|||
| Line 5: | Line 5: | ||
== Type signatures and ambiguity == | == Type signatures and ambiguity == | ||
| + | |||
| + | 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: | ||
| + | <haskell> | ||
| + | class C a b where | ||
| + | foo :: a -> b | ||
| + | |||
| + | konst :: a -> Bool | ||
| + | konst x = True | ||
| + | |||
| + | f :: (C a b) => a -> Bool | ||
| + | f x = konst (foo x) | ||
| + | </haskell> | ||
| + | If you comment out the type signature <hask>f :: (C a b) => a -> Bool</hask>, the module will load fine into GHCi, and <tt>:t</tt> will report exactly this type for <tt>f</tt>. But if you leave the type signature in, 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:12:13-17 | ||
| + | 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? Without the type signature, GHC picks a 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 abouut <tt>b</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>. That would be OK if there was an instance like: | ||
| + | <haskell> | ||
| + | instance C Char b where ... | ||
| + | </haskell> | ||
== Overlapping instances == | == Overlapping instances == | ||
Revision as of 14:25, 20 February 2007
Type system extensions in GHC
GHC comes with a rather large collection of type-system extensions (beyond Haskell 98). They are all documented in the user manual, but this page is a place to record observations, notes, and suggestions on them.
Contents |
1 Type signatures and ambiguity
It's quite common for people to write a function definition without a type signature, load it into GHCi, use :t 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:
class C a b where foo :: a -> b konst :: a -> Bool konst x = True f :: (C a b) => a -> Bool f x = konst (foo x)
Foo1.hs:12:13:
Could not deduce (C a b1) from the context (C a b)
arising from use of `foo' at Foo1.hs:12:13-17
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)
What's going on? Without the type signature, GHC picks a type for x, say x::a. Then applying foo means GHC must pick a return type for foo, say b, and generates the type constraint (C a b). The function konst just discards its argument, so nothing further is known abouut b. So GHC ends up saying that This is probably a very stupid type. Suppose you called f thus: (f 'a'). Then you'd get a constraint (C Char b) where nothing is known about b. That would be OK if there was an instance like:
instance C Char b where ...
2 Overlapping instances
Here an interesting message about the interaction of existential types and overlapping instances.
3 Indexed data types and indexed newtypes
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 its source repository.
4 Stand-alone deriving clauses
Bjorn Bringert has recently implemented "stand-alone deriving" declarations.
