I am writing a test for a binary search function I wrote.
module Tests where
import Data.List (sort)
import Test.QuickCheck
import BinarySearch (binarySearch)
prop_equals_elem x xs = (binarySearch x $ sort xs) == (x `elem` xs)
args = Args {replay = Nothing, maxSuccess = 200, maxDiscard=200, maxSize=200, chatty = False}
main = do
quickCheck (prop_equals_elem :: (Ord a) => a -> [a] -> Bool)
It works well using quickCheck in ghci but when I try to run main it gives the error
Tests.hs:12:5:
Ambiguous type variable `a0' in the constraints:
(Arbitrary a0) arising from a use of `quickCheckWith'
at Tests.hs:12:5-18
(Show a0) arising from a use of `quickCheckWith'
at Tests.hs:12:5-18
(Ord a0) arising from an expression type signature
at Tests.hs:12:26-72
Why does this not work in main but does in ghci?
This is likely caused by the extended defaulting rules in GHCi.
When testing a function like this, you need to use a concrete element type. GHCi will default the element type to () because of the extended rules, but this will not happen when compiling the code normally, so GHC is telling you that it cannot figure out which element type to use.
You can for example use Int instead for the test. () is pretty useless for testing this function, as all elements would be the same.
quickCheck (prop_equals_elem :: Int -> [Int] -> Bool)
If it works for Int, it should work for any type due to parametricity.
When you run a QuickCheck test, QuickCheck needs to know how to generate data. Here, you've told it only that your code should work with an arbitrary type of the Ord type class, which isn't enough for it to start testing. Hence the error about ambiguous type classes.
If you just need an arbitrary Ord instance, as it appears here, then something like Int would be a good choice for your testing. It's a simple type with a linear order. So try fixing your type to Int in main, as in:
quickCheck (prop_equals_elem :: Int -> [Int] -> Bool)
As for why it works in GHCi, the answer is defaulting. GHCi defaults type variables to () whenever possible, just to avoid giving spurious errors in situations where you really don't care about a value. Actually that's an awful choice: you won't test anything interesting by only testing with the () type! So again, the explicit type signature is better.
Related
This example works with ghci, load this file:
import Safe
t1 = tailMay []
and put in ghci:
> print t1
Nothing
But if we add analogous definition to previous file, it doesn't work:
import Safe
t1 = tailMay []
t2 = print $ tailMay []
with such error:
* Ambiguous type variable `a0' arising from a use of `print'
prevents the constraint `(Show a0)' from being solved.
Probable fix: use a type annotation to specify what `a0' should be.
These potential instances exist:
instance Show Ordering -- Defined in `GHC.Show'
instance Show Integer -- Defined in `GHC.Show'
instance Show a => Show (Maybe a) -- Defined in `GHC.Show'
...plus 22 others
That is 3rd sample for ghc with the same error:
import Safe
t1 = tailMay
main = do
print $ t1 []
print $ t1 [1,2,3]
Why? And how to fix the second sample without explicit type annotation?
The issue here is that tailMay [] can generate an output of type Maybe [a] for any a, while print can take an input of type Maybe [a] for any a (in class Show).
When you compose a "universal producer" and a "universal consumer", the compiler has no idea about which type a to pick -- that could be any type in class Show. The choice of a could matter since, in principle, print (Nothing :: Maybe [Int]) could print something different from print (Nothing :: Maybe [Bool]). In this case, the printed output would be the same, but only because we are lucky.
For instance print ([] :: [Int]) and print ([] :: [Char]) will print different messages, so print [] is ambiguous. Hence, GHC reject it, and requires an explicit type annotation (or a type application # type, using an extension).
Why, then, such ambiguity is accepted in GHCi? Well, GHCi is meant to be used for quick experiments, and as such, as a convenience feature, it will try hard to default these ambiguous a. This is done using the extended defaulting rules, which could (I guess) in principle be turned on in GHC as well by turning on that extension.
This is, however, not recommended since sometimes the defaulting rule can choose some unintended type, making the code compile but with an unwanted runtime behavior.
The common solution to this issue is using an annotation (or # type), because it provides more control to the programmer, makes the code easier to read, and avoids surprises.
This question already has answers here:
How to reuse a type variable in an inner type declaration
(4 answers)
Arrays with rigid variable
(2 answers)
Closed 8 years ago.
The following is a simplified example of what I'm trying to do...
test :: Bounded a => Maybe a -> a
test (Just x) = x
test Nothing = (maxBound :: a)
The maxBound function is polymorphic - one of the methods of the Bounded typeclass. Because of that, when I use it I need to specify which version of Bounded I want. In this simplified example, that type could be inferred from the context - but in the real problem it can't - the explicit type is necessary in the real problem, though not really here.
My function is polymorphic too. I can't specify a concrete type directly, only a type variable. The appropriate type variable is a, for which I have specified the Bounded a constraint.
Compiling this, I get the following error...
temp.hs:4:18:
Could not deduce (Bounded a1) arising from a use of `maxBound'
from the context (Bounded a)
bound by the type signature for test :: Bounded a => Maybe a -> a
at temp.hs:2:9-33
Possible fix:
add (Bounded a1) to the context of
an expression type signature: a1
or the type signature for test :: Bounded a => Maybe a -> a
In the expression: (maxBound :: a)
In an equation for `test': test Nothing = (maxBound :: a)
As far as I can tell, this means that the a in maxBound :: a was considered separate from the a that I intended (the type variable in the signature for the function). a1 is the new name that GHC invented to disambiguate the two a variables which it considers separate. GHC considers the a in maxBound :: a to indicate that it can use any type here (!) and therefore complains because "any type" isn't restrictive enough.
This is using GHC version 7.6.3 as supplied in the (I think) most recent Haskell Platform.
I've had similar issues before, but always mixed with other issues, so the problem went away once I fixed those other problems. I dismissed it as being caused by the other issues and forgot about it. No such luxury here - that minimal example above isn't the real problem, but it depends on a solution to the exact same problem.
So... why is GHC treating the a in maxBound :: a as independent of the type variable a for the whole function? And how do I fix this to select the correct version of maxBound?
Main problem is in fact, that GHC tried to see function as
test :: forall a. Bounded a => Maybe a -> a
test (Just x) = x
test Nothing = (maxBound :: forall a. a)
You need ScopedTypeVariables extension and rewrite function to:
{-# LANGUAGE ScopedTypeVariables #-}
test :: forall a. Bounded a => Maybe a -> a
test (Just x) = x
test Nothing = (maxBound :: a)
Now we see, that inner a is depend on outer a
UPDATE
If you already wrote the signatiure, you don't need any extension.
Next function works fine!
test :: Bounded a => Maybe a -> a
test (Just x) = x
test Nothing = maxBound
I am having trouble with ambiguous types in Haskell. I started out with the following:
module GameState
( GameState(..)
, GameStateMonad
, module Control.Monad.Trans
, module Control.Monad.Trans.State.Lazy
, Blank(..)
) where
import Control.Monad.Trans
import Control.Monad.Trans.State.Lazy
type GameStateMonad a b = StateT a IO b
class GameState a where
update :: Double -> GameStateMonad a ()
update deltaTime = return ()
draw :: GameStateMonad a ()
draw = return ()
getNextState :: GameState b => GameStateMonad a (Maybe b)
getNextState = return Nothing
isStateFinished :: GameStateMonad a Bool
isStateFinished = return True
-- This is just a dummy data and instance declaration to demonstrate the error
data Blank = Blank
instance GameState Blank
Then when I try to run the following in ghci:
runStateT getNextState Blank
I get:
Ambiguous type variable `b0' in the constraint:
(GameState b0) arising from a use of `getNextState'
Probable fix: add a type signature that fixes these type variable(s)
...
I thought it was complaining that my default implementation of the getNextState function didn't specify a concrete type, so I tried the following:
getNextState :: GameState b => GameStateMonad a (Maybe b)
getNextState = return (Nothing :: Maybe Blank)
Unfortunately I got this error while compiling:
Could not deduce (b ~ Blank)
from the context (GameState a)
bound by the class declaration for `GameState'
at GameState.hs:(14,1)-(25,33)
or from (GameState b)
bound by the type signature for
getNextState :: GameState b => GameStateMonad a (Maybe b)
at GameState.hs:22:5-50
`b' is a rigid type variable bound by
the type signature for
getNextState :: GameState b => GameStateMonad a (Maybe b)
at GameState.hs:22:5
...
But I found that adding a type signature when I call getNext state allows the code to run:
runStateT (getNextState :: GameStateMonad Blank (Maybe Blank)) Blank
Unfortunately this stops me from making generic code to handle game states. It also makes little sense to me. What's the point in returning a polymorphic type if you have to give it an explicit type after it's returned? The original issue is also really confusing to me because I can make a function as follows:
test :: Num a => Maybe a
test = Nothing
And have no problems running it. Shouldn't this complain about ambiguous types like my original code? Also when giving the return value an explicit type I cannot compile it, like before:
test :: Num a => Maybe a
test = Nothing :: Maybe Int
I don't see why this is a problem. Int is an instance of type Num so the type of the function is correct.
I have four questions:
Why does giving an explicit type when returning an element of a typeclass cause a compile error?
Why does returning an ambiguous Maybe value inside of getNextState cause an error but inside test it does not?
Why does this error occur without me calling a function on the returned polymorphic data, as explained here?
In the link above, the answer mentions that "[you get this error] because you have something that produces a polymorphic result, then apply a function that takes a polymorphic argument to that result, such that the intermediate value's type is unknown". Doesn't this mean that functions that return a polymorphic result are essentially useless?
Thanks.
Cat Plus Plus has already explained why
getNextState :: GameState b => GameStateMonad a (Maybe b)
getNextState = return (Nothing :: Maybe Blank)
doesn't work, so I can be short on that. The type signature promises that getNextState can deliver a value of type Maybe b for whatever type b the caller demands. It's the caller of the function who decides what type it shall return if a function has polymorphic return type. So the signature promises "whatever you want, as long as it's a GameState instance", but the implementation says "No, I don't care what you ordered, I return a Blank".
Ambiguous type variable `b0' in the constraint:
(GameState b0) arising from a use of `getNextState'
Probable fix: add a type signature that fixes these type variable(s)
from typing
runStateT getNextState Blank
at the ghci prompt. If you ask ghci for the type of that, it will tell you
runStateT getNextState Blank :: GameState b => IO (Maybe b)
(no guarantees about the choice of type variable). But there is no context, so ghci doesn't know which type to instantiate b with. And so it doesn't know which implementation of getNextState it should call [or, which dictionary it should pass, if we look at GHC's implementation of type classes]. It has no way of resolving that ambiguity, so it tells you about it and suggests how you could resolve it.
test :: Num a => Maybe a
test = Nothing
Yes, that's in principle the same problem, when you type test at the ghci prompt. But there are special rules for resolving ambiguous type variables when there is one numeric class involved (where ambiguities are most frequent, literals are already ambiguous), and all involved constraints are simple and concern classes of the Prelude or the standard libraries. In that case, ambiguous type variables are instantiated by defaulting, so ghci would choose to instantiate a with Integer and print the Nothing of the type Maybe Integer.
Your GameState class is not defaultable, that's the difference between these examples.
Why does this error occur without me calling a function on the returned polymorphic data, as explained here?
Because you're not calling any function that would determine the type. If you had a function of type
foo :: Blank -> Int
and typed
runStateT getNextState Blank >>= print . maybe 0 foo
the use of foo would determine b and all would be swell.
The problem would however not be solved but exacerbated, if you called a polymorphic function (whose argument type cannot be inferred from its result type), as in the linked example. Then the ambiguous type is no longer reachable from the outside, and it can never be resolved. Then the only way is to provide a type signature that resolves the ambiguity.
Doesn't this mean that functions that return a polymorphic result are essentially useless?
Oh no, they are immensely useful. Look at read, or fromInteger, realToFrac, ...
The point is, that the type at which they shall be used must somehow be determined where they are used. Most of the time that is done by the calling context, but sometimes an explicit type signature is necessary.
I'm not sure what you're trying to achieve by making GameState a typeclass. You might be too much in OOP mindset here — typeclasses are not OOP classes. A set of game states will probably be closed, so it might make more sense to just make it a single ADT.
Why does giving an explicit type when returning an element of a typeclass cause a compile error?
Look at your function's signature: GameState b => GameStateMonad a (Maybe b). Now remember that this implies forall b. GameState b => GameStateMonad a (Maybe b).
Implementation of the function cannot decide what b is — it must work for all of them (as long as they fit in constraints), because it's up for the caller to decide.
That's why return (Nothing :: Maybe Blank) is an error — it doesn't work for all types b — it only works for Blank. "Could not deduce (b ~ Blank)" means GHC can't prove type equality here. Same thing goes for Nothing :: Maybe Int. And that's also why adding type signature in the call site works.
I'll write something on ambiguities later maybe. I'm still pretty sure you're overengineering this code anyway, so the solution is to not do that.
I have a little problem to understand an error message in haskell.
For instance:
import qualified Data.Map as M
test = M.empty
This code runs as it should do without getting any error message.
The output looks like:
*Main> test
fromList []
But if I try something like that
import qualified Data.Map as M
test = do print M.empty
I get an error message like this
Ambiguous type variable `k0' in the constraint:
(Show k0) arising from a use of `print'
Probable fix: add a type signature that fixes these type variable(s)
In a stmt of a 'do' block: print M.empty
In the expression: do { print M.empty }
In an equation for `test': test = do { print M.empty }
So I think it has something to do with the print statement.
But if I try it in the console (ghci)
Prelude Data.Map> print empty
fromList []
everything works fine.
So I hope someone can explain me where the problem is.
Thanks in advance.
This code runs as it should do without getting any error message.
In a source file, it shouldn't.
import qualified Data.Map as M
test = M.empty
The inferred type of test is Ord k => Map k a, a polymorphic type with a constrained type variable. Since test is not a function and has no type signature, by the monomorphism restriction, its type must be made monomorphic by resolving the constrained type variables to a default type. Since the only constraint here is Ord, the defaulting rules forbid that type variable to be defaulted (there must be at least one numeric constraint for defaulting to be allowed).
Thus, compilation is required to fail by the language standard.
In ghci, however, there are extended defaulting rules that allow to default the type. If you want to print test, a further Show constraint is introduced on both type variables, and ghci defaults the type of test to Map () () when asked to print it.
This is because Data.Map.empty has the type Map k a. A map of keys of type k to values, type a.
print on the other hand has the type print :: Show a => a -> IO (), which means that it can only display types that are instances of Show, while M.empty has type Map k a, has no such constraint. It can be any type k or a -- it is not required to be able to show them.
So, basically, print doesn't know what type it's being asked to display.
As for why it works in ghci; I'm not entirely sure. Maybe one of the resident Haskell wizards can shed some light on that.
One could think of this case as follows:
The application dynamically loads a module, or there is a list of functions from which the user chooses, etc. We have a mechanism for determining whether a certain type will successfully work with a function in that module. So now we want to call into that function. We need to force it to make the call. The function could take a concrete type, or a polymorphic one and it's the case below with just a type class constraint that I'm running into problems with it.
The following code results in the errors below. I think it could be resolved by specifying concrete types but I do not want to do that. The code is intended to work with any type that is an instance of the class. Specifying a concrete type defeats the purpose.
This is simulating one part of a program that does not know about the other and does not know the types of what it's dealing with. I have a separate mechanism that allows me to be sure that the types do match up properly, that the value sent in really is an instance of the type class. That's why in this case, I don't mind using unsafeCoerce. But basically I need a way to tell the compiler that I really do know it's ok and do it anyway even though it doesn't know enough to type check.
{-# LANGUAGE ExistentialQuantification, RankNTypes, TypeSynonymInstances #-}
module Main where
import Unsafe.Coerce
main = do
--doTest1 $ Hider "blue"
doTest2 $ Hider "blue"
doTest1 :: Hider -> IO ()
doTest1 hh#(Hider h) =
test $ unsafeCoerce h
doTest2 :: Hider -> IO ()
doTest2 hh#(Hider h) =
test2 hh
test :: HasString a => a -> IO ()
test x = print $ toString x
test2 :: Hider -> IO ()
test2 (Hider x) = print $ toString (unsafeCoerce x)
data Hider = forall a. Hider a
class HasString a where
toString :: a -> String
instance HasString String where
toString = id
Running doTest1
[1 of 1] Compiling Main ( Test.hs, Test.o )
Test.hs:12:3:
Ambiguous type variable `a1' in the constraint:
(HasString a1) arising from a use of `test'
Probable fix: add a type signature that fixes these type variable(s)
In the expression: test
In the expression: test $ unsafeCoerce h
In an equation for `doTest1':
doTest1 hh#(Hider h) = test $ unsafeCoerce h
Running doTest2
[1 of 1] Compiling Main ( Test.hs, Test.o )
Test.hs:12:3:
Ambiguous type variable `a1' in the constraint:
(HasString a1) arising from a use of `test'
Probable fix: add a type signature that fixes these type variable(s)
In the expression: test
In the expression: test $ unsafeCoerce h
In an equation for `doTest1':
doTest1 hh#(Hider h) = test $ unsafeCoerce h
I think it could be resolved by specifying concrete types but I do not want to do that.
There's no way around it though with unsafeCoerce. In this particular case, the compiler can't infer the type of unsafeCoerce, because test is still to polymorphic. Even though there is just one instance of HasString, the type system won't use that fact to infer the type.
I don't have enough information about your particular application of this pattern, but I'm relatively sure that you need to rethink the way you use the type system in your program. But if you really want to do this, you might want to look into Data.Typeable instead of unsafeCoerce.
Modify your data type slightly:
data Hider = forall a. HasString a => Hider a
Make it an instance of the type class in the obvious way:
instance HasString Hider where
toString (Hider x) = toString x
Then this should work, without use of unsafeCoerce:
doTest3 :: Hider -> IO ()
doTest3 hh = print $ toString hh
This does mean that you can no longer place a value into a Hider if it doesn't implement HasString, but that's probably a good thing.
There's probably a name for this pattern, but I can't think what it is off the top of my head.