#1571: type of synthesize in Data.Generics.Schemes is too restrictive
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Reporter: [EMAIL PROTECTED] | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: libraries/base | Version: 6.6.1
Severity: normal | Keywords: synthesize,syb
Difficulty: Easy (1 hr) | Os: Linux
Testcase: | Architecture: x86
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The type of the synthesize function in Data.Generics.Schemes is
unnecessarily restrictive. It's current type is
synthesize :: s -> (s -> s -> s) -> GenericQ (s -> s) -> GenericQ s
but it would be more useful if it were
synthesize :: s -> (t -> s -> s) -> GenericQ (s -> t) -> GenericQ t
Below is a contrived example demonstrating why one might want the more
liberal type.
module Main where
import Data.Generics
synthesize' :: s -> (t -> s -> s) -> GenericQ (s -> t) -> GenericQ t
synthesize' z o f x = f x (foldr o z (gmapQ (synthesize' z o f) x))
-- The toTree function fails to type if synthesize' is replaced
-- with synthesize.
data ConstructorTree = ConstructorTree String [ConstructorTree] deriving
(Show)
toTree :: Data a => a -> ConstructorTree
toTree = synthesize' [] (:) (\a s -> ConstructorTree (showConstr (toConstr
a)) s)
data Foo = Bar String | Baz Foo Int deriving (Data,Typeable)
main = print (toTree (Baz (Bar "12") 5))
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1571>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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