In my actual problem I have a function f
, passed as a parameter, that changes the order in a list but has no requirements regarding type and does not change the type either. I want to apply that function on [Int]
and [Bool]
so I have to resolve both contexts trying to typecast f
to either [Int] -> [Int]
or [Bool] -> [Bool]
. I solved this with Rank2Types
. But then I use any
on a list of functions like f
and any
requires the functions to be [a] -> [a]
rather than forall a. [a] -> [a]
forall a. [a] -> [a]
.
The code below while nonsensical perfectly reproduces the error:
{-# LANGUAGE Rank2Types #-}
--debug :: (forall a. [a] -> [a]) -> Bool
debug swap = any combine [swap]
where
combine :: (forall a. [a] -> [a]) -> Bool
combine f = usefonBool f && usefonInt f
--usefonBool :: (forall a. [a] -> [a]) -> Bool
usefonBool f = f [True,True] == [False]
--usefonInt :: (forall a. [a] -> [a]) -> Bool
usefonInt f = (f [1,2]) == [2,1]
The error message is:
• Couldn't match type ‘a’ with ‘forall a1. [a1] -> [a1]’
‘a’ is a rigid type variable bound by
the inferred type of debug :: a -> Bool
at /path/debug.hs:(4,1)-(12,36)
Expected type: a -> Bool
Actual type: (forall a. [a] -> [a]) -> Bool
• In the first argument of ‘any’, namely ‘combine’
In the expression: any combine [swap]
In an equation for ‘debug’:
debug swap
= any combine [swap]
where
combine :: (forall a. [a] -> [a]) -> Bool
combine f = usefonBool f && usefonInt f
usefonBool f = f [True, ....] == [False]
usefonInt f = (f [1, ....]) == [2, ....]
• Relevant bindings include
swap :: a (bound at /path/debug.hs:4:7)
debug :: a -> Bool (bound at /path/debug.hs:4:1)
|
My goal is to find an annotation that lets me use an f
on different types and then apply any on a list of such generic functions.
If I uncomment all my type annotations (or just the top one) the error changes to
• Couldn't match type ‘[a0] -> [a0]’ with ‘forall a. [a] -> [a]’
Expected type: ([a0] -> [a0]) -> Bool
Actual type: (forall a. [a] -> [a]) -> Bool
• In the first argument of ‘any’, namely ‘combine’
In the expression: any combine [swap]
In an equation for ‘debug’:
debug swap
= any combine [swap]
where
combine :: (forall a. [a] -> [a]) -> Bool
combine f = usefonBool f && usefonInt f
usefonBool :: (forall a. [a] -> [a]) -> Bool
usefonBool f = f [True, ....] == [False]
....
|
First, note that Rank2Types
is a deprecated name. It's equivalent to RankNTypes
in modern GHC, and that's the preferred name for the extension.
Here's the underlying problem. A "list of such generic functions" could have the type:
[forall a. [a] -> [a]]
Sadly, this isn't a valid Haskell type because Haskell doesn't support "impredicative polymorphism". Specifically, the following program:
{-# LANGUAGE RankNTypes #-}
myFunctions :: [forall a. [a] -> [a]]
myFunctions = [f1, f2]
where f1 (x:y:rest) = y:x:rest
f2 = reverse
produces the error message:
DebugRank.hs:2:16: error: • Illegal polymorphic type: forall a. [a] -> [a] GHC doesn't yet support impredicative polymorphism • In the type signature: myFunctions :: [forall a. [a] -> [a]]
There's an extension, ImpredicativeTypes
. It's flakey and incomplete, but it allows the following to compile:
{-# LANGUAGE Rank2Types #-}
{-# LANGUAGE ImpredicativeTypes #-}
myFunctions :: [forall a. [a] -> [a]]
myFunctions = [f1, f2]
where f1 (x:y:rest) = y:x:rest
f2 = reverse
debug :: [forall a. [a] -> [a]] -> Bool
debug = any combine
where
combine :: (forall a. [a] -> [a]) -> Bool
combine f = usefonBool f && usefonInt f
usefonBool f = f [True,True] == [False]
usefonInt f = f [1,2] == [2,1]
main = print (debug myFunctions)
I'd still recommend against using it, though.
The usual alternative is to use a newtype
wrapper for the polymorphic function:
newtype ListFunction = ListFunction (forall a. [a] -> [a])
This requires some boilerplate, but no extensions other than RankNTypes
:
myFunctions :: [ListFunction]
myFunctions = [ListFunction f1, ListFunction f2]
where f1 (x:y:rest) = y:x:rest
f2 = reverse
debug :: [ListFunction] -> Bool
debug = any combine
where
combine :: ListFunction -> Bool
combine (ListFunction f) = usefonBool f && usefonInt f
usefonBool f = f [True,True] == [False]
usefonInt f = f [1,2] == [2,1]
The complete code:
{-# LANGUAGE RankNTypes #-}
newtype ListFunction = ListFunction (forall a. [a] -> [a])
myFunctions :: [ListFunction]
myFunctions = [ListFunction f1, ListFunction f2]
where f1 (x:y:rest) = y:x:rest
f2 = reverse
debug :: [ListFunction] -> Bool
debug = any combine
where
combine :: ListFunction -> Bool
combine (ListFunction f) = usefonBool f && usefonInt f
usefonBool f = f [True,True] == [False]
usefonInt f = f [1,2] == [2,1]
main = print $ debug myFunctions
The technical post webpages of this site follow the CC BY-SA 4.0 protocol. If you need to reprint, please indicate the site URL or the original address.Any question please contact:yoyou2525@163.com.