Difference between revisions of "GHC/AdvancedOverlap"
(→Choosing a typeclass instance based on the context: minor whitespace issue) 
(fix the whitespace issue consistently through the whole document) 

Line 5:  Line 5:  
Suppose you have this class: 
Suppose you have this class: 

<haskell> 
<haskell> 

−  +  class Print a where 

print :: a > IO () 
print :: a > IO () 

</haskell> 
</haskell> 

Line 23:  Line 23:  
First define an auxiliary class Print': 
First define an auxiliary class Print': 

<haskell> 
<haskell> 

−  +  class Print' flag a where 

print' :: flag > a > IO () 
print' :: flag > a > IO () 

−  +  instance (ShowPred a flag, Print' flag a) => Print a 

print = print' (undefined::flag) 
print = print' (undefined::flag) 

</haskell> 
</haskell> 

Line 33:  Line 33:  
precisely mirror those of Show: 
precisely mirror those of Show: 

<haskell> 
<haskell> 

−  +  class ShowPred a flag  a>flag where {} 

 Used only if the other 
 Used only if the other 

 instances don't apply 
 instances don't apply 

−  +  instance TypeCast flag HFalse => ShowPred a flag 

−  +  instance ShowPred Int HTrue  These instances must be 

−  +  instance ShowPred Bool HTrue  the same as Show's 

−  +  instance ShowPred a flag => ShowPred [a] flag 

−  +  ...etc... 

−  +  data HTrue  Just two 

−  +  data HFalse  distinct types 

</haskell> 
</haskell> 

These instances do make use of overlapping instances, but they do not 
These instances do make use of overlapping instances, but they do not 

Line 99:  Line 99:  
which has the auxiliary flag: 
which has the auxiliary flag: 

<haskell> 
<haskell> 

−  +  class Show' a flag  a>flag where 

show :: a > String 
show :: a > String 

 This instance is used only if the others don't apply 
 This instance is used only if the others don't apply 

−  +  instance TypeCast flag HFalse => Show' a flag where 

show = error "urk" 
show = error "urk" 

 These instances are the regular ones 
 These instances are the regular ones 

−  +  instance Show' Int HTrue where 

show = showInt 
show = showInt 

−  +  instance Show' Bool HTrue where 

show = showBool 
show = showBool 

−  +  ...etc... 

</haskell> 
</haskell> 

Now we can write the instances for Print': 
Now we can write the instances for Print': 

Line 131:  Line 131:  
class And a b c  a b > c 
class And a b c  a b > c 

−  +  instance And HTrue b b 

−  +  instance And HFalse b HFalse 

</haskell> 
</haskell> 

The HList paper shows many examples of such typelevel programming. 
The HList paper shows many examples of such typelevel programming. 

Line 138:  Line 138:  
4. Using type families, we'd like to express it like this: 
4. Using type families, we'd like to express it like this: 

<haskell> 
<haskell> 

−  +  class Print' flag a where 

print' :: a > IO () 
print' :: a > IO () 

−  +  instance Print' (ShowWorks a) a => Print a 

print = print' 
print = print' 

−  +   ShowWorks is a predicate on types, which says 

−  +   which ones are instances of class Show 

−  +  type family ShowWorks a 

−  +  type instance ShowWorks a = HFalse 

−  +  type instance ShowPred Int = HTrue 

−  +  type instance ShowPred Bool = HTrue 

−  +  type instance ShowWorks [a] = ShowWorks a 

−  +  type instance ShowWorks (a,b) = And (ShowWorks a, ShowWorks b) 

−  +  ...etc... 

instance (Show a) => Print' HTrue a where 
instance (Show a) => Print' HTrue a where 
Revision as of 05:09, 10 August 2011
Choosing a typeclass instance based on the context
Oleg Kiselyov and Simon PeytonJones (Apr 2008)
Suppose you have this class:
class Print a where
print :: a > IO ()
Now suppose you want to say "if type a is in class Show, print one way, otherwise print another way". You'd probably try to write this:
instance Show a => Print a where
print x = putStrLn (show x)
instance Print a where
print x = putStrLn "No show method"
But that is illegal in Haskell, because the heads of the two instance declarations are identical. Nevertheless, you can code it up using functional dependencies and overlapping instances, and that's what this note describes.
First define an auxiliary class Print':
class Print' flag a where
print' :: flag > a > IO ()
instance (ShowPred a flag, Print' flag a) => Print a
print = print' (undefined::flag)
The main class Print has only one instance, and there is no longer any overlapping. The new class ShowPred has no methods, but its instances precisely mirror those of Show:
class ShowPred a flag  a>flag where {}
 Used only if the other
 instances don't apply
instance TypeCast flag HFalse => ShowPred a flag
instance ShowPred Int HTrue  These instances must be
instance ShowPred Bool HTrue  the same as Show's
instance ShowPred a flag => ShowPred [a] flag
...etc...
data HTrue  Just two
data HFalse  distinct types
These instances do make use of overlapping instances, but they do not rely on the *context* to distinguish which one to pick, just the instance *head*. Notice that (ShowPred ty flag) always succeeds! If <ty> is a type for which there is a Show instance, flag gets unified to HTrue; otherwise flag gets unified to HFalse.
Now we can write the (nonoverlapping) instances for Print':
instance (Show a) => Print' HTrue a where
print' _ x = putStrLn (show x)
instance Print' HFalse a where
print' _ x = putStrLn "No show method"
The trick is to rewrite a constraint (C a) which succeeds of fails, into a predicate constraint (C' a flag), which always succeeds, but once discharged, unifies flag with either HTrue or HFalse. The desired invariant is
C a succeeds <> C' a flag unifies flag with HTrue
Perhaps the most puzzling is the constraint (TypeCast flag HFalse) in the first instance of ShowPred. The TypeCast constraint and its important role are explained in Section 9 and specifically Appendix D of the full HList paper <http://homepages.cwi.nl/~ralf/HList/paper.pdf>
Notes and variations
1. A more `closed world' alternative: write ShowPred as follows
> class ShowPred a flag  a>flag where {}
> instance HMember a Showtypes flag => ShowPred a flag
There is only one instance of ShowPred and there is no overlapping instances. Here, Showtypes are defined as
> type Showtypes = Int :+: Bool :+: Char :+: ... :+: HNil
(Polymorphic types like [a] take more effort, but they too can be handled). This is the closed list of types, and HMember is a HList membership checker. HMember uses TypeEq  and the latter is the only place that requires overlapping instances.
2. There is, of course, no check that the instances of ShowPred match those of Show; you just have to get that right. An alternative, which trades this problem for another, is instead to *replace* by Show', which has the auxiliary flag:
class Show' a flag  a>flag where
show :: a > String
 This instance is used only if the others don't apply
instance TypeCast flag HFalse => Show' a flag where
show = error "urk"
 These instances are the regular ones
instance Show' Int HTrue where
show = showInt
instance Show' Bool HTrue where
show = showBool
...etc...
Now we can write the instances for Print':
instance Show' HTrue a => Print' HTrue a where
print' x = putStrLn (show x)
instance Print' HFalse a where
print' x = putStrLn "No show method"
The disadvantage here is, of course, that you have to change the Show class.
3. We need a bit of boolean algebra in the more interesting instances
of ShowPred:
instance (ShowPred a flag1, ShowPred b flag2, And flag1 flag2 flag)
=> (ShowPred (a,b) flag
class And a b c  a b > c
instance And HTrue b b
instance And HFalse b HFalse
The HList paper shows many examples of such typelevel programming.
4. Using type families, we'd like to express it like this:
class Print' flag a where
print' :: a > IO ()
instance Print' (ShowWorks a) a => Print a
print = print'
 ShowWorks is a predicate on types, which says
 which ones are instances of class Show
type family ShowWorks a
type instance ShowWorks a = HFalse
type instance ShowPred Int = HTrue
type instance ShowPred Bool = HTrue
type instance ShowWorks [a] = ShowWorks a
type instance ShowWorks (a,b) = And (ShowWorks a, ShowWorks b)
...etc...
instance (Show a) => Print' HTrue a where
print' x = putStrLn (show x)
instance Print' HFalse a where
print' x = putStrLn "No show method"
However, there's a problem: overlap is not allowed at all for type families!! There is a good reason for this, but it's not helpful here.
Appendix: the sample code
{# LANGUAGE EmptyDataDecls,
MultiParamTypeClasses,
ScopedTypeVariables,
FunctionalDependencies,
OverlappingInstances,
FlexibleInstances,
UndecidableInstances #}
module Main where
import Prelude hiding (print)
class Print a where
print :: a > IO ()
{ the following does not work:
instance Show a => Print a where
print x = putStrLn (show x)
instance Print a where
print x = putStrLn "No show method"
error:
Duplicate instance declarations:
instance (Show a) => Print a  Defined at /tmp/wiki.hs:7:0
instance Print a  Defined at /tmp/wiki.hs:9:0
}
class Print' flag a where
print' :: flag > a > IO ()
instance (ShowPred a flag, Print' flag a) => Print a where
print = print' (undefined::flag)
 overlapping instances are used only for ShowPred
class ShowPred a flag  a>flag where {}
 Used only if the other
 instances don't apply
instance TypeCast flag HFalse => ShowPred a flag
instance ShowPred Int HTrue  These instances should be
instance ShowPred Bool HTrue  the same as Show's
instance ShowPred a flag => ShowPred [a] flag
 ...etc...
data HTrue  Just two
data HFalse  distinct types
instance Show a => Print' HTrue a where
print' _ x = putStrLn (show x)
instance Print' HFalse a where
print' _ x = putStrLn "No show method"
test1 = print [True,False]  [True,False]
test2 = print id  No show method
 see http://okmij.org/ftp/Haskell/typecast.html
class TypeCast a b  a > b, b>a where typeCast :: a > b
class TypeCast' t a b  t a > b, t b > a where typeCast' :: t>a>b
class TypeCast'' t a b  t a > b, t b > a where typeCast'' :: t>a>b
instance TypeCast' () a b => TypeCast a b where typeCast x = typeCast' () x
instance TypeCast'' t a b => TypeCast' t a b where typeCast' = typeCast''
instance TypeCast'' () a a where typeCast'' _ x = x