In some languages (#racket/typed, for example), the programmer can specify a union type without discriminating against it, for instance, the type (U Integer String) captures integers and strings, without tagging them (I Integer) (S String) in a data IntOrStringUnion = ... form or anything like that.
Is there a way to do the same in Haskell?
Either is what you're looking for... ish.
In Haskell terms, I'd describe what you're looking for as an anonymous sum type. By anonymous, I mean that it doesn't have a defined name (like something with a data declaration). By sum type, I mean a data type that can have one of several (distinguishable) types; a tagged union or such. (If you're not familiar with this terminology, try Wikipedia for starters.)
We have a well-known idiomatic anonymous product type, which is just a tuple. If you want to have both an Int and a String, you just smush them together with a comma: (Int, String). And tuples (seemingly) can go on forever--(Int, String, Double, Word), and you can pattern-match the same way. (There's a limit, but never mind.)
The well-known idiomatic anonymous sum type is Either, from Data.Either (and the Prelude):
data Either a b = Left a | Right b
deriving (Eq, Ord, Read, Show, Typeable)
It has some shortcomings, most prominently a Functor instance that favors Right in a way that's odd in this context. The problem is that chaining it introduces a lot of awkwardness: the type ends up like Either (Int (Either String (Either Double Word))). Pattern matching is even more awkward, as others have noted.
I just want to note that we can get closer to (what I understand to be) the Racket use case. From my extremely brief Googling, it looks like in Racket you can use functions like isNumber? to determine what type is actually in a given value of a union type. In Haskell, we usually do that with case analysis (pattern matching), but that's awkward with Either, and function using simple pattern-matching will likely end up hard-wired to a particular union type. We can do better.
IsNumber?
I'm going to write a function I think is an idiomatic Haskell stand-in for isNumber?. First, we don't like doing Boolean tests and then running functions that assume their result; instead, we tend to just convert to Maybe and go from there. So the function's type will end with -> Maybe Int. (Using Int as a stand-in for now.)
But what's on the left hand of the arrow? "Something that might be an Int -- or a String, or whatever other types we put in the union." Uh, okay. So it's going to be one of a number of types. That sounds like typeclass, so we'll put a constraint and a type variable on the left hand of the arrow: MightBeInt a => a -> Maybe Int. Okay, let's write out the class:
class MightBeInt a where
isInt :: a -> Maybe Int
fromInt :: Int -> a
Okay, now how do we write the instances? Well, we know if the first parameter to Either is Int, we're golden, so let's write that out. (Incidentally, if you want a nice exercise, only look at the instance ... where parts of these next three code blocks, and try to implement that class members yourself.)
instance MightBeInt (Either Int b) where
isInt (Left i) = Just i
isInt _ = Nothing
fromInt = Left
Fine. And ditto if Int is the second parameter:
instance MightBeInt (Either a Int) where
isInt (Right i) = Just i
isInt _ = Nothing
fromInt = Right
But what about Either String (Either Bool Int)? The trick is to recurse on the right hand type: if it's not Int, is it an instance of MightBeInt itself?
instance MightBeInt b => MightBeInt (Either a b) where
isInt (Right xs) = isInt xs
isInt _ = Nothing
fromInt = Right . fromInt
(Note that these all require FlexibleInstances and OverlappingInstances.) It took me a long time to get a feel for writing and reading these class instances; don't worry if this instance is surprising. The punch line is that we can now do this:
anInt1 :: Either Int String
anInt1 = fromInt 1
anInt2 :: Either String (Either Int Double)
anInt2 = fromInt 2
anInt3 :: Either String Int
anInt3 = fromInt 3
notAnInt :: Either String Int
notAnInt = Left "notint"
ghci> isInt anInt3
Just 3
ghci> isInt notAnInt
Nothing
Great!
Generalizing
Okay, but now do we need to write another type class for each type we want to look up? Nope! We can parameterize the class by the type we want to look up! It's a pretty mechanical translation; the only question is how to tell the compiler what type we're looking for, and that's where Proxy comes to the rescue. (If you don't want to install tagged or run base 4.7, just define data Proxy a = Proxy. It's nothing special, but you'll need PolyKinds.)
class MightBeA t a where
isA :: proxy t -> a -> Maybe t
fromA :: t -> a
instance MightBeA t t where
isA _ = Just
fromA = id
instance MightBeA t (Either t b) where
isA _ (Left i) = Just i
isA _ _ = Nothing
fromA = Left
instance MightBeA t b => MightBeA t (Either a b) where
isA p (Right xs) = isA p xs
isA _ _ = Nothing
fromA = Right . fromA
ghci> isA (Proxy :: Proxy Int) anInt3
Just 3
ghci> isA (Proxy :: Proxy String) notAnInt
Just "notint"
Now the usability situation is... better. The main thing we've lost, by the way, is the exhaustiveness checker.
Notational Parity With (U String Int Double)
For fun, in GHC 7.8 we can use DataKinds and TypeFamilies to eliminate the infix type constructors in favor of type-level lists. (In Haskell, you can't have one type constructor--like IO or Either--take a variable number of parameters, but a type-level list is just one parameter.) It's just a few lines, which I'm not really going to explain:
type family OneOf (as :: [*]) :: * where
OneOf '[] = Void
OneOf '[a] = a
OneOf (a ': as) = Either a (OneOf as)
Note that you'll need to import Data.Void. Now we can do this:
anInt4 :: OneOf '[Int, Double, Float, String]
anInt4 = fromInt 4
ghci> :kind! OneOf '[Int, Double, Float, String]
OneOf '[Int, Double, Float, String] :: *
= Either Int (Either Double (Either Float [Char]))
In other words, OneOf '[Int, Double, Float, String] is the same as Either Int (Either Double (Either Float [Char])).
You need some kind of tagging because you need to be able to check if a value is actually an Integer or a String to use it for anything. One way to alleviate having to create a custom ADT for every combination is to use a type such as
{-# LANGUAGE TypeOperators #-}
data a :+: b = L a | R b
infixr 6 :+:
returnsIntOrString :: Integer -> Integer :+: String
returnsIntOrString i
| i `rem` 2 == 0 = R "Even"
| otherwise = L (i * 2)
returnsOneOfThree :: Integer -> Integer :+: String :+: Bool
returnsOneOfThree i
| i `rem` 2 == 0 = (R . L) "Even"
| i `rem` 3 == 0 = (R . R) False
| otherwise = L (i * 2)
Related
Exploring and studing type system in Haskell I've found some problems.
1) Let's consider polymorphic type as Binary Tree:
data Tree a = Leaf a | Branch (Tree a) (Tree a) deriving Show
And, for example, I want to limit my considerations only with Tree Int, Tree Bool and Tree Char. Of course, I can make a such new type:
data TreeIWant = T1 (Tree Int) | T2 (Tree Bool) | T3 (Tree Char) deriving Show
But could it possible to make new restricted type (for homogeneous trees) in more elegant (and without new tags like T1,T2,T3) way (perhaps with some advanced type extensions)?
2) Second question is about trees with heterogeneous values. I can do them with usual Haskell, i.e. I can do the new helping type, contained tagged heterogeneous values:
data HeteroValues = H1 Int | H2 Bool | H3 Char deriving Show
and then make tree with values of this type:
type TreeH = Tree HeteroValues
But could it possible to make new type (for heterogeneous trees) in more elegant (and without new tags like H1,H2,H3) way (perhaps with some advanced type extensions)?
I know about heterogeneous list, perhaps it is the same question?
For question #2, it's easy to construct a "restricted" heterogeneous type without explicit tags using a GADT and a type class:
{-# LANGUAGE GADTs #-}
data Thing where
T :: THING a => a -> Thing
class THING a
Now, declare THING instances for the the things you want to allow:
instance THING Int
instance THING Bool
instance THING Char
and you can create Things and lists (or trees) of Things:
> t1 = T 'a' -- Char is okay
> t2 = T "hello" -- but String is not
... type error ...
> tl = [T (42 :: Int), T True, T 'x']
> tt = Branch (Leaf (T 'x')) (Leaf (T False))
>
In terms of the type names in your question, you have:
type HeteroValues = Thing
type TreeH = Tree Thing
You can use the same type class with a new GADT for question #1:
data ThingTree where
TT :: THING a => Tree a -> ThingTree
and you have:
type TreeIWant = ThingTree
and you can do:
> tt1 = TT $ Branch (Leaf 'x') (Leaf 'y')
> tt2 = TT $ Branch (Leaf 'x') (Leaf False)
... type error ...
>
That's all well and good, until you try to use any of the values you've constructed. For example, if you wanted to write a function to extract a Bool from a possibly boolish Thing:
maybeBool :: Thing -> Maybe Bool
maybeBool (T x) = ...
you'd find yourself stuck here. Without a "tag" of some kind, there's no way of determining if x is a Bool, Int, or Char.
Actually, though, you do have an implicit tag available, namely the THING type class dictionary for x. So, you can write:
maybeBool :: Thing -> Maybe Bool
maybeBool (T x) = maybeBool' x
and then implement maybeBool' in your type class:
class THING a where
maybeBool' :: a -> Maybe Bool
instance THING Int where
maybeBool' _ = Nothing
instance THING Bool where
maybeBool' = Just
instance THING Char where
maybeBool' _ = Nothing
and you're golden!
Of course, if you'd used explicit tags:
data Thing = T_Int Int | T_Bool Bool | T_Char Char
then you could skip the type class and write:
maybeBool :: Thing -> Maybe Bool
maybeBool (T_Bool x) = Just x
maybeBool _ = Nothing
In the end, it turns out that the best Haskell representation of an algebraic sum of three types is just an algebraic sum of three types:
data Thing = T_Int Int | T_Bool Bool | T_Char Char
Trying to avoid the need for explicit tags will probably lead to a lot of inelegant boilerplate elsewhere.
Update: As #DanielWagner pointed out in a comment, you can use Data.Typeable in place of this boilerplate (effectively, have GHC generate a lot of boilerplate for you), so you can write:
import Data.Typeable
data Thing where
T :: THING a => a -> Thing
class Typeable a => THING a
instance THING Int
instance THING Bool
instance THING Char
maybeBool :: Thing -> Maybe Bool
maybeBool = cast
This perhaps seems "elegant" at first, but if you try this approach in real code, I think you'll regret losing the ability to pattern match on Thing constructors at usage sites (and so having to substitute chains of casts and/or comparisons of TypeReps).
I am currently working my way through Write Yourself a Scheme in 48 Hours and am stuck on type promotion.
In short, scheme has a numerical tower (Integer->Rational->Real->Complex) with the numeric promotions one would expect. I modeled numbers with the obvious
data Number = Integer Integer | Rational Rational | Real Double | Complex (Complex Double)
so using Rank2Types seemed like an easy way to make functions work over this range of types. For Num a this looks like
liftNum :: (forall a . Num a => a -> a -> a) -> LispVal -> LispVal -> ThrowsError LispVal
liftNum f a b = case typeEnum a `max` typeEnum b of
ComplexType -> return . Number . Complex $ toComplex a `f` toComplex b
RealType -> return . Number . Real $ toReal a `f` toReal b
RationalType -> return . Number . Rational $ toFrac a `f` toFrac b
IntType -> return . Number . Integer $ toInt a `f` toInt b
_ -> typeErr a b "Number"
which works but quickly becomes verbose because a separate block is required for each type class.
Even worse, this implementation of Complex is simplified since scheme can use separate types for the real and complex part. Implementing this would require a custom version holding two Number's which would make the verbosity even worse if I wanted to avoid making the type recursive.
As far as I know there is no way to abstract over the context so I am hoping for a cleaner way to implement this number logic.
Thanks for reading!
Here's a proposal. The primary thing we want your typeEnum function to do that it doesn't yet is bring a Num a dictionary into scope. So let's use GADTs to make that happen. I'll simplify a few things to make it easier to explain the idea and write the code, but nothing essential: I'll focus on Number rather than LispVal and I won't report detailed errors when things go wrong. First some boilerplate:
{-# LANGUAGE GADTs #-}
{-# LANGUAGE Rank2Types #-}
import Control.Applicative
import Data.Complex
Now, you didn't give your definition of a type enumeration. But I'll give mine, because it's part of the secret sauce: my type enumeration is going to have a connection between Haskell's term level and Haskell's type level via a GADT.
data TypeEnum a where
Integer :: TypeEnum Integer
Rational :: TypeEnum Rational
Real :: TypeEnum Double
Complex :: TypeEnum (Complex Double)
Because of this connection, my Number type won't need to repeat each of these cases again. (I suspect your TypeEnum and Number types are quite repetitive compared to each other.)
data Number where
Number :: TypeEnum a -> a -> Number
Now we're going to define a new type that you didn't have, which will tie a TypeEnum together with a Num dictionary for the appropriate type. This will be the return type of our typeEnum function.
data TypeDict where
TypeDict :: Num a => TypeEnum a -> TypeDict
ordering :: TypeEnum a -> Int
ordering Integer = 0 -- lowest
ordering Rational = 1
ordering Real = 2
ordering Complex = 3 -- highest
instance Eq TypeDict where TypeDict l == TypeDict r = ordering l == ordering r
instance Ord TypeDict where compare (TypeDict l) (TypeDict r) = compare (ordering l) (ordering r)
The ordering function reflects (my guess at) the direction that casts can go. If you try to implement Eq and Ord yourself for this type, without peeking at my solution, I suspect you will discover why GHC balks at deriving these classes for GADTs. (At the very least, it took me a few tries! The obvious definitions don't type-check, and the slightly less obvious definitions had the wrong behavior.)
Now we are ready to write a function that produces a dictionary for a number.
typeEnum :: Number -> TypeDict
typeEnum (Number Integer _) = TypeDict Integer
typeEnum (Number Rational _) = TypeDict Rational
typeEnum (Number Real _) = TypeDict Real
typeEnum (Number Complex _) = TypeDict Complex
We will also need the casting function; you can essentially just concatenate your definitions of toComplex and friends here:
-- combines toComplex, toFrac, toReal, toInt
to :: TypeEnum a -> Number -> Maybe a
to Rational (Number Integer n) = Just (fromInteger n)
to Rational (Number Rational n) = Just n
to Rational _ = Nothing
-- etc.
to _ _ = Nothing
Once we have this machinery in place, liftNum is surprisingly short. We just find the appropriate type to cast to, get a dictionary for that type, and perform the casts and the operation.
liftNum :: (forall a. Num a => a -> a -> a) -> Number -> Number -> Maybe Number
liftNum f a b = case typeEnum a `max` typeEnum b of
TypeDict ty -> Number ty <$> liftA2 f (to ty a) (to ty b)
At this point you may be complaining: your ultimate goal was to not have one case per class instance in liftNum, and we've achieved that goal, but it looks like we just pushed it off into the definition of typeEnum, where there is one case per class instance. However, I defend myself: you have not shown us your typeEnum, which I suspect already had one case per class instance. So we have not incurred any new cost in functions other than liftNum, and have indeed significantly simplified liftNum. This also gives a smooth upgrade path for more complex Complex manipulations: extend the TypeEnum definition, the cast ordering, and the to function and you're good to go; liftNum may stay the same. (If it turns out that types are not linearly ordered but instead some kind of lattice or similar, then you can switch away from the Ord class.)
I have a question pertaining to deserialization. I can envision a solution using Data.Data, Data.Typeable, or with GHC.Generics, but I'm curious if it can be accomplished without generics, SYB, or meta-programming.
Problem Description:
Given a list of [String] that is known to contain the fields of a locally defined algebraic data type, I would like to deserialize the [String] to construct the target data type. I could write a parser to do this, but I'm looking for a generalized solution that will deserialize to an arbitrary number of data types defined within the program without writing a parser for each type. With knowledge of the number and type of value constructors an algebraic type has, it's as simple as performing a read on each string to yield the appropriate values necessary to build up the type. However, I don't want to use generics, reflection, SYB, or meta-programming (unless it's otherwise impossible).
Say I have around 50 types defined similar to this (all simple algebraic types composed of basic primitives (no nested or recursive types, just different combinations and orderings of primitives) :
data NetworkMsg = NetworkMsg { field1 :: Int, field2 :: Int, field3 :: Double}
data NetworkMsg2 = NetworkMsg2 { field1 :: Double, field2 :: Int, field3 :: Double }
I can determine the data-type to be associated with a [String] I've received over the network using a tag id that I parse before each [String].
Possible conjectured solution path:
Since data constructors are first-class values in Haskell, and actually have a type-- Can NetworkMsg constructor be thought of as a function, such as:
NetworkMsg :: Int -> Int -> Double -> NetworkMsg
Could I transform this function into a function on tuples using uncurryN then copy the [String] into a tuple of the same shape the function now takes?
NetworkMsg' :: (Int, Int, Double) -> NetworkMsg
I don't think this would work because I'd need knowledge of the value constructors and type information, which would require Data.Typeable, reflection, or some other metaprogramming technique.
Basically, I'm looking for automatic deserialization of many types without writing type instance declarations or analyzing the type's shape at run-time. If it's not feasible, I'll do it an alternative way.
You are correct in that the constructors are essentially just functions so you can write generic instances for any number of types by just writing instances for the functions. You'll still need to write a separate instance
for all the different numbers of arguments, though.
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
import Text.Read
import Control.Applicative
class FieldParser p r where
parseFields :: p -> [String] -> Maybe r
instance Read a => FieldParser (a -> r) r where
parseFields con [a] = con <$> readMaybe a
parseFields _ _ = Nothing
instance (Read a, Read b) => FieldParser (a -> b -> r) r where
parseFields con [a, b] = con <$> readMaybe a <*> readMaybe b
parseFields _ _ = Nothing
instance (Read a, Read b, Read c) => FieldParser (a -> b -> c -> r) r where
parseFields con [a, b, c] = con <$> readMaybe a <*> readMaybe b <*> readMaybe c
parseFields _ _ = Nothing
{- etc. for as many arguments as you need -}
Now you can use this type class to parse any message based on the constructor as long as the type-checker is able to figure out the resulting message type from context (i.e. it is not able to deduce it simply from the given constructor for these sort of multi-param type class instances).
data Test1 = Test1 {fieldA :: Int} deriving Show
data Test2 = Test2 {fieldB ::Int, fieldC :: Float} deriving Show
test :: String -> [String] -> IO ()
test tag fields = case tag of
"Test1" -> case parseFields Test1 fields of
Just (a :: Test1) -> putStrLn $ "Succesfully parsed " ++ show a
Nothing -> putStrLn "Parse error"
"Test2" -> case parseFields Test2 fields of
Just (a :: Test2) -> putStrLn $ "Succesfully parsed " ++ show a
Nothing -> putStrLn "Parse error"
I'd like to know how exactly you use the message types in the application, though, because having each message as its separate type makes it very difficult to have any sort of generic message handler.
Is there some reason why you don't simply have a single message data type? Such as
data NetworkMsg
= NetworkMsg1 {fieldA :: Int}
| NetworkMsg2 {fieldB :: Int, fieldC :: Float}
Now, while the instances are built in pretty much the same way, you get much better type inference since the result type is always known.
instance Read a => MessageParser (a -> NetworkMsg) where
parseMsg con [a] = con <$> readMaybe a
instance (Read a, Read b) => MessageParser (a -> b -> NetworkMsg) where
parseMsg con [a, b] = con <$> readMaybe a <*> readMaybe b
instance (Read a, Read b, Read c) => MessageParser (a -> b -> c -> NetworkMsg) where
parseMsg con [a, b, c] = con <$> readMaybe a <*> readMaybe b <*> readMaybe c
parseMessage :: String -> [String] -> Maybe NetworkMsg
parseMessage tag fields = case tag of
"NetworkMsg1" -> parseMsg NetworkMsg1 fields
"NetworkMsg2" -> parseMsg NetworkMsg2 fields
_ -> Nothing
I'm also not sure why you want to do type-generic programming specifically without actually using any of the tools meant for generics. GHC.Generics, SYB or Template Haskell is usually the best solution for this kind of problem.
(For the following, simplify Show and Read to
class Show a where show :: a -> String
class Read a where read :: String -> a
And assume that read never fails.)
It's well-known that one can make an existential type of the form
data ShowVal where
ShowVal :: forall a. Show a => a -> ShowVal
And then construct a "heterogeneous list" :: [ShowVal], such as
l = [ShowVal 4, ShowVal 'Q', ShowVal True]
It's also well-known that this is relatively useless, because, instead, one can
just construct a list :: [String], such as
l = [show 4, show 'Q', show True]
Which is exactly isomorphic (after all, the only thing one can do with a
ShowVal is show it).
Laziness makes this particularly nice, because for each value in the list, the
result of show is memoized automatically, so no String is computed more than
once (and Strings that aren't used aren't computed at all).
A ShowVal is equivalent to an existential tuple exists a. (a -> String, a),
where the function is the Show dictionary.
A similar construct can be made for Read:
data ReadVal where
ReadVal :: (forall a. Read a => a) -> ReadVal
Note that, because read is polymorphic in its return value, ReadVal is
universal rather than existential (which means that we don't really need it at
all, because Haskell has first-class universals; but we'll use it here to
highlight the similaries to Show).
We can also make a list :: [ReadVal]:
l = [ReadVal (read "4"), ReadVal (read "'Q'"), ReadVal (read "True")]
Just as with Show, a list :: [ReadVal] is isomorphic to a list :: [String],
such as
l = ["4", "'Q'", "True"]
(We can always get the original String back with
newtype Foo = Foo String
instance Read Foo where read = Foo
Because the Read type class is open.)
A ReadVal is equivalent to a universal function forall a. (String -> a) -> a
(a CPS-style representation). Here the Read dictionary is supplied by the user
of the ReadVal rather than by the producer, because the return value is
polymorphic rather than the argument.
However, in neither of these representations do we get the automatic
memoization that we get in the String representation with Show. Let's say that
read for our type is an expensive operation, so we don't want to compute it
on the same String for the same type more than once.
If we had a closed type, we could do something like:
data ReadVal = ReadVal { asInt :: Int, asChar :: Char, asBool :: Bool }
And then use a value
ReadVal { asInt = read s, asChar = read s, asBool = read s }
Or something along those lines.
But in this case -- even if we only ever use the ReadVal as one type -- the
String will be parsed each time the value is used. Is there a simple way to
get memoization while keeping the ReadVal polymorphic?
(Getting GHC to do it automatically, similarly to the Show case, would be
ideal, if it's somehow possible. A more explicit memoization approach --
perhaps by adding a Typeable constraint? -- would also be OK.)
Laziness makes this particularly nice, because for each value in the list, the result of show is memoized automatically, so no String is computed more than once (and Strings that aren't used aren't computed at all).
This premise is incorrect. There is no magical memo table under the hood.
Laziness means things that aren't needed, aren't computed. It does not mean that all computed values are shared. You still have to introduce explicit sharing (via a table of your own).
Here's an implementation of the more explicit approach; it requires Typeable, because otherwise there'd be nothing to key the memo table on. I based the memoisation code on uglymemo; there might be a way to get this to work with pure memoisation, but I'm not sure. It's tricky, because you have to construct the table outside of the implicit function that any forall a. (Read a, Typeable a) => ... creates, otherwise you end up constructing one table per call, which is useless.
{-# LANGUAGE GADTs, RankNTypes #-}
import Data.Dynamic
import Control.Concurrent.MVar
import Data.HashMap.Strict (HashMap)
import qualified Data.HashMap.Strict as HM
import System.IO.Unsafe
data ReadVal where
ReadVal :: { useReadVal :: forall a. (Read a, Typeable a) => a } -> ReadVal
mkReadVal :: String -> ReadVal
mkReadVal s = unsafePerformIO $ do
v <- newMVar HM.empty
return $ ReadVal (readVal v)
where
readVal :: (Read a, Typeable a) => MVar (HashMap TypeRep Dynamic) -> a
readVal v = unsafePerformIO $ do
m <- readMVar v
let r = read s -- not evaluated
let typeRep = typeOf r
case HM.lookup typeRep m of
Nothing -> do
modifyMVar_ v (return . HM.insert typeRep (toDyn r))
return r
Just r' -> return $ fromDyn r' (error "impossible")
I have a type
class IntegerAsType a where
value :: a -> Integer
data T5
instance IntegerAsType T5 where value _ = 5
newtype (IntegerAsType q) => Zq q = Zq Integer deriving (Eq)
newtype (Num a, IntegerAsType n) => PolyRing a n = PolyRing [a]
I'm trying to make a nice "show" for the PolyRing type. In particular, I want the "show" to print out the type 'a'. Is there a function that returns the type of an algebraic parameter (a 'show' for types)?
The other way I'm trying to do it is using pattern matching, but I'm running into problems with built-in types and the algebraic type.
I want a different result for each of Integer, Int and Zq q.
(toy example:)
test :: (Num a, IntegerAsType q) => a -> a
(Int x) = x+1
(Integer x) = x+2
(Zq x) = x+3
There are at least two different problems here.
1) Int and Integer are not data constructors for the 'Int' and 'Integer' types. Are there data constructors for these types/how do I pattern match with them?
2) Although not shown in my code, Zq IS an instance of Num. The problem I'm getting is:
Ambiguous constraint `IntegerAsType q'
At least one of the forall'd type variables mentioned by the constraint
must be reachable from the type after the '=>'
In the type signature for `test':
test :: (Num a, IntegerAsType q) => a -> a
I kind of see why it is complaining, but I don't know how to get around that.
Thanks
EDIT:
A better example of what I'm trying to do with the test function:
test :: (Num a) => a -> a
test (Integer x) = x+2
test (Int x) = x+1
test (Zq x) = x
Even if we ignore the fact that I can't construct Integers and Ints this way (still want to know how!) this 'test' doesn't compile because:
Could not deduce (a ~ Zq t0) from the context (Num a)
My next try at this function was with the type signature:
test :: (Num a, IntegerAsType q) => a -> a
which leads to the new error
Ambiguous constraint `IntegerAsType q'
At least one of the forall'd type variables mentioned by the constraint
must be reachable from the type after the '=>'
I hope that makes my question a little clearer....
I'm not sure what you're driving at with that test function, but you can do something like this if you like:
{-# LANGUAGE ScopedTypeVariables #-}
class NamedType a where
name :: a -> String
instance NamedType Int where
name _ = "Int"
instance NamedType Integer where
name _ = "Integer"
instance NamedType q => NamedType (Zq q) where
name _ = "Zq (" ++ name (undefined :: q) ++ ")"
I would not be doing my Stack Overflow duty if I did not follow up this answer with a warning: what you are asking for is very, very strange. You are probably doing something in a very unidiomatic way, and will be fighting the language the whole way. I strongly recommend that your next question be a much broader design question, so that we can help guide you to a more idiomatic solution.
Edit
There is another half to your question, namely, how to write a test function that "pattern matches" on the input to check whether it's an Int, an Integer, a Zq type, etc. You provide this suggestive code snippet:
test :: (Num a) => a -> a
test (Integer x) = x+2
test (Int x) = x+1
test (Zq x) = x
There are a couple of things to clear up here.
Haskell has three levels of objects: the value level, the type level, and the kind level. Some examples of things at the value level include "Hello, world!", 42, the function \a -> a, or fix (\xs -> 0:1:zipWith (+) xs (tail xs)). Some examples of things at the type level include Bool, Int, Maybe, Maybe Int, and Monad m => m (). Some examples of things at the kind level include * and (* -> *) -> *.
The levels are in order; value level objects are classified by type level objects, and type level objects are classified by kind level objects. We write the classification relationship using ::, so for example, 32 :: Int or "Hello, world!" :: [Char]. (The kind level isn't too interesting for this discussion, but * classifies types, and arrow kinds classify type constructors. For example, Int :: * and [Int] :: *, but [] :: * -> *.)
Now, one of the most basic properties of Haskell is that each level is completely isolated. You will never see a string like "Hello, world!" in a type; similarly, value-level objects don't pass around or operate on types. Moreover, there are separate namespaces for values and types. Take the example of Maybe:
data Maybe a = Nothing | Just a
This declaration creates a new name Maybe :: * -> * at the type level, and two new names Nothing :: Maybe a and Just :: a -> Maybe a at the value level. One common pattern is to use the same name for a type constructor and for its value constructor, if there's only one; for example, you might see
newtype Wrapped a = Wrapped a
which declares a new name Wrapped :: * -> * at the type level, and simultaneously declares a distinct name Wrapped :: a -> Wrapped a at the value level. Some particularly common (and confusing examples) include (), which is both a value-level object (of type ()) and a type-level object (of kind *), and [], which is both a value-level object (of type [a]) and a type-level object (of kind * -> *). Note that the fact that the value-level and type-level objects happen to be spelled the same in your source is just a coincidence! If you wanted to confuse your readers, you could perfectly well write
newtype Huey a = Louie a
newtype Louie a = Dewey a
newtype Dewey a = Huey a
where none of these three declarations are related to each other at all!
Now, we can finally tackle what goes wrong with test above: Integer and Int are not value constructors, so they can't be used in patterns. Remember -- the value level and type level are isolated, so you can't put type names in value definitions! By now, you might wish you had written test' instead:
test' :: Num a => a -> a
test' (x :: Integer) = x + 2
test' (x :: Int) = x + 1
test' (Zq x :: Zq a) = x
...but alas, it doesn't quite work like that. Value-level things aren't allowed to depend on type-level things. What you can do is to write separate functions at each of the Int, Integer, and Zq a types:
testInteger :: Integer -> Integer
testInteger x = x + 2
testInt :: Int -> Int
testInt x = x + 1
testZq :: Num a => Zq a -> Zq a
testZq (Zq x) = Zq x
Then we can call the appropriate one of these functions when we want to do a test. Since we're in a statically-typed language, exactly one of these functions is going to be applicable to any particular variable.
Now, it's a bit onerous to remember to call the right function, so Haskell offers a slight convenience: you can let the compiler choose one of these functions for you at compile time. This mechanism is the big idea behind classes. It looks like this:
class Testable a where test :: a -> a
instance Testable Integer where test = testInteger
instance Testable Int where test = testInt
instance Num a => Testable (Zq a) where test = testZq
Now, it looks like there's a single function called test which can handle any of Int, Integer, or numeric Zq's -- but in fact there are three functions, and the compiler is transparently choosing one for you. And that's an important insight. The type of test:
test :: Testable a => a -> a
...looks at first blush like it is a function that takes a value that could be any Testable type. But in fact, it's a function that can be specialized to any Testable type -- and then only takes values of that type! This difference explains yet another reason the original test function didn't work. You can't have multiple patterns with variables at different types, because the function only ever works on a single type at a time.
The ideas behind the classes NamedType and Testable above can be generalized a bit; if you do, you get the Typeable class suggested by hammar above.
I think now I've rambled more than enough, and likely confused more things than I've clarified, but leave me a comment saying which parts were unclear, and I'll do my best.
Is there a function that returns the type of an algebraic parameter (a 'show' for types)?
I think Data.Typeable may be what you're looking for.
Prelude> :m + Data.Typeable
Prelude Data.Typeable> typeOf (1 :: Int)
Int
Prelude Data.Typeable> typeOf (1 :: Integer)
Integer
Note that this will not work on any type, just those which have a Typeable instance.
Using the extension DeriveDataTypeable, you can have the compiler automatically derive these for your own types:
{-# LANGUAGE DeriveDataTypeable #-}
import Data.Typeable
data Foo = Bar
deriving Typeable
*Main> typeOf Bar
Main.Foo
I didn't quite get what you're trying to do in the second half of your question, but hopefully this should be of some help.