I'm learning Haskell and I've decided to to the H-99 problem set. Naturally, I've become stuck on the first problem!
I have the following code:
module Main where
getLast [] = []
getLast x = x !! ((length x) - 1)
main = do
putStrLn "Enter a list:"
x <- readLn
print (getLast x)
Compiling this code gives the following error:
h-1.hs:8:14:
No instance for (Read a0) arising from a use of `readLn'
The type variable `a0' is ambiguous
Possible fix: add a type signature that fixes these type variable(s)
Note: there are several potential instances:
instance Read () -- Defined in `GHC.Read'
instance (Read a, Read b) => Read (a, b) -- Defined in `GHC.Read'
instance (Read a, Read b, Read c) => Read (a, b, c)
-- Defined in `GHC.Read'
...plus 25 others
In a stmt of a 'do' block: x <- readLn
In the expression:
do { putStrLn "Enter a list:";
x <- readLn;
print (getLast x) }
In an equation for `main':
main
= do { putStrLn "Enter a list:";
x <- readLn;
print (getLast x) }
h-1.hs:9:9:
No instance for (Show a0) arising from a use of `print'
The type variable `a0' is ambiguous
Possible fix: add a type signature that fixes these type variable(s)
Note: there are several potential instances:
instance Show Double -- Defined in `GHC.Float'
instance Show Float -- Defined in `GHC.Float'
instance (Integral a, Show a) => Show (GHC.Real.Ratio a)
-- Defined in `GHC.Real'
...plus 26 others
In a stmt of a 'do' block: print (getLast x)
In the expression:
do { putStrLn "Enter a list:";
x <- readLn;
print (getLast x) }
In an equation for `main':
main
= do { putStrLn "Enter a list:";
x <- readLn;
print (getLast x) }
That's a large error, but it seems to me that Haskell isn't sure what the input type will be. That's fine, and completely understandable. However, as this is supposed to work on a list of generics, I'm not sure how to specify that type. I tried this:
x :: [a] <- readLn
...as [a] is the type that Haskell returns for an empty list (found with :t []). This won't compile either.
As I'm a beginner, I know there's a lot I'm missing, but in a basic sense - how can I satisfy Haskell's type system with input code? I'm a Haskell beginner looking for a beginner answer, if that's at all possible. (Also, note that I know there's a better way to do this problem (reverse, head) but this is the way I came up with first, and I'd like to see if I can make it work.)
You can't hope to write something like this which will detect the type of x at run time -- what kind of thing you're reading must be known at compile time. That's why #Sibi's answer uses [Int]. If it can't be deduced, you get a compile time error.
If you want a polymorphic read, you have to construct your own parser which lists the readable types.
maybeDo :: (Monad m) => Maybe a -> (a -> m b) -> m b
maybeDo f Nothing = return ()
maybeDo f (Just x) = f x
main = do
str <- getLine
maybeDo (maybeRead str :: Maybe Int) $ \i ->
putStrLn $ "Got an Int: " ++ show i
maybeDo (maybeRead str :: Maybe String) $ \s ->
putStrLn $ "Got a String: " ++ show s
There are lots of ways to factor out this repetition, but at some point you'll have to list all the types you'll accept.
(An easy way to see the problem is to define a new type MyInt which has the same Read instance as Int -- then how do we know whether read "42" should return an Int or a MyInt?)
This should work:
getLast :: Num a => [a] -> a
getLast [] = 0
getLast x = x !! ((length x) - 1)
main = do
putStrLn "Enter a list:"
x <- readLn :: IO [Int]
print (getLast x)
why return 0 for an empty list, instead of an empty list?
Because it won't typecheck. Because you are returning [] for empty list and for other cases you are returning the element inside the list i.e a. Now since a type is not equal to list, it won't typecheck. A better design would be to catch this type of situation using the Maybe datatype.
Also, because of returning 0, the above function will work only for List of types which have Num instances created for them. You can alleviate that problem using error function.
However, this should work for a generic list, not just a list of Ints
or Numbers, right?
Yes, it should work for a polymorphic list. And you can create a function like getLast which will work for all type of List. But when you want to get input from the user, it should know what type of input you are giving. Because the typechecker won't be able to know whether you meant it as List of Int or List of Double or so on.
Related
Say I have the following function:
readList :: IO [Int]
readList = do
putStrLn "Please enter the list as a string"
putStrLn "Example: input of '1 2 3 4 5' will map to [1,2,3,4,5]"
line <- getLine
return $ map read $ words line
printNaive :: [Int] -> IO ()
printNaive xs = putStrLn "The maximum surpasser count is:" >> putStrLn "0"
main :: IO ()
main = readList >>= printNaive
This function works as expected. Now lets say I was going to extend this code to be more generic, and read in a line of any type of thing as a list:
readList :: (Read a, Int a) -> IO [a]
readList = do
putStrLn "Please enter the list as a string"
putStrLn "Example: input of '1 2 3 4 5' will map to [1,2,3,4,5]"
line <- getLine
return $ map read $ words line
printNaive :: (Eq a) => [a] -> IO ()
printNaive xs = putStrLn "The maximum surpasser count is:" >> putStrLn "0"
main :: IO ()
main = readList >>= printNaive
This fails with:
Ambiguous type variable ‘a0’ arising from a use of ‘Lib.readList’
prevents the constraint ‘(Read a0)’ from being solved.
Probable fix: use a type annotation to specify what ‘a0’ should be.
These potential instances exist:
instance Read Ordering -- Defined in ‘GHC.Read’
instance Read Integer -- Defined in ‘GHC.Read’
instance Read a => Read (Maybe a) -- Defined in ‘GHC.Read’
...plus 22 others
...plus four instances involving out-of-scope types
(use -fprint-potential-instances to see them all)
How would I go about writing this code, given I really don't care what type of thing it is as long as it conforms to Eq.
Additionally, say I wanted to provide a facility to specify what type the list is going to contain. (through another getLine say).
How would I extract a type from getLine, and how would I then cast every element in the map read $ words line to that particular type.
What you did wrong is in this line:
readList :: (Read a, Int a) -> IO [a]
What you probably want to get at with (Read a, Int a) is a type class constranit for the type a, meaning you want it to be readable and you want it to be some sort of integer.
Firstly you wrote your constraint wrong. A typeclass constraint is given before a => not a ->. Secondly Int is not a type class. Maybe try using Integral?
So your type signature should look like this:
readList :: (Read a, Integral a) => IO [a]
EDIT: A caveat is, however, that the a in the type signature has to be decided at compile time. In the first example in your question it would work out because the type of printNaive fixes a to be Int. That is not generally the case, however.
I try to write a quick sort in Haskell and I knew there are many versions out there.
This one is pretty simple for Int type
quickSort1::[Int]->[Int]
quickSort1 [] = []
quickSort1 (x:xs) = [l | l <- xs, l < x] ++ [x] ++ [ r | r <- xs, r > x]
I can print on Main as following
print $ quickSort1 [] -- output []
print $ quickSort1 [2, 1] -- output [1, 2]
I modified the above quickSort1 to "more general" type with (Ord a) instead of Int
quickSort2::(Ord a)=>[a]->Maybe [a]
quickSort2 [] = Nothing
quickSort2 (x:xs) = Just $ [ l | l <- xs, l < x] ++ [x] ++ [ r | r <- xs, r > x]
On my Main, I can run
it works
print $ quickSort2 [2, 1] -- output [1, 2]
I got compiler error when I run following
print $ quickSort2 [] -- got error
Can anyone explain to me what is going on with my new version of quickSort2
I assume you use a file foo.hs and in it
main = print $ quicksort []
quicksort = ... - as defined above in quickSort2
then you get two error messages when you runghc foo.hs
foo.hs:3:8: 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
...plus 11 instances involving out-of-scope types
(use -fprint-potential-instances to see them all)
• In the expression: print $ quicksort []
In an equation for ‘main’: main = print $ quicksort []
one telling you that ghc cannot tell what Show instance to use and ghc 8 already tells you how to solve this:
add a type annotation (as #duplode already suggested)
main = print $ quicksort ([] :: [Int])
Quite similar but slightly different is the second error message
foo.hs:3:16: error:
• Ambiguous type variable ‘a0’ arising from a use of ‘quicksort’
prevents the constraint ‘(Ord a0)’ from being solved.
Probable fix: use a type annotation to specify what ‘a0’ should be.
These potential instances exist:
instance Ord Ordering -- Defined in ‘GHC.Classes’
instance Ord Integer
-- Defined in ‘integer-gmp-1.0.0.1:GHC.Integer.Type’
instance Ord a => Ord (Maybe a) -- Defined in ‘GHC.Base’
...plus 22 others
...plus five instances involving out-of-scope types
(use -fprint-potential-instances to see them all)
• In the second argument of ‘($)’, namely ‘quicksort []’
In the expression: print $ quicksort []
In an equation for ‘main’: main = print $ quicksort []
Where in the first message the print function demanded a Show instance - here you promised the quicksort to supply a list of orderables - but did not say which to use, so GHC complains about what Ord to use.
Both messages are due to the fact that [] is too polymorphic it could be a list of anything - [Int] is good, but it could also be something like [Int -> Bool] which is neither Showable nor Orderable.
You could as well supply quicksort with something weird like a
newtype HiddenInt = HI Int deriving (Ord) --but not Show
which would work for the quicksort function but not for print.
Side Note
Your quicksort functions need to be recursive in order to really be correct - as I pointed out in my comments - there is a logical problem in your algorithm - be sure to test your functions properly e.g.
import Data.List (sort)
main :: IO ()
main = do print $ "quicksort [10,9..1] == Just (sort [10,9..1]) is: "
++ show $ quicksort ([10,9..1]::Int]) == Just (sort ([10,9..1]::Int]))
print $ "quicksort [5,5,5] == Just (sort [5,5,5]) is: "
++ show $ quicksort ([5,5,5] :: [Int]) == Just (sort ([5,5,5] :: [Int]))
quicksort :: (Ord a) => [a] -> Maybe [a]
quicksort = ...
or if you are interested take a look at QuickCheck - which is a bit more advanced, but a step in the right direction for verifying your algorithms/functions work the way you expect them.
main = do
putStrLn $myLast [1,2,3,4]
myLast :: [a] -> a
myLast [x] = x
myLast (_:xs) = myLast xs
When i try to run this code i get this message:
"No instance for (Num String) arising from the literal `1'
Possible fix: add an instance declaration for (Num String)"
It runs well when I run with the list ["1","2","3,"4"]. I didn't specify the type but it doesn't work with ints.
"No instance for..." error messages are usually misleading.
The problem you have is simply this
Prelude> :t putStrLn
putStrLn :: String -> IO ()
i.e. that function can only deal with strings, not with numbers. An often-seen solution is to first translate the thing you want to show into a string: putStrLn (show x), but actually the combination exists as a much nicer standard function:
main = do
print $ myLast [1,2,3,4]
The compiler concludes from
putStrLn x
that x must be a String. The inferred type for
myLast [1,2,3,4]
is Num a => a and when you now substitute a with String you get
Num String => String
This is all quite logical, except that the type checker remembers that the Num constraint originated from the literal 1.
The message you get is thus just another way to say that a number is not a string, and putStrLn badly wants a string. Or, if you want, that the expression would be well typed, if only strings were numbers.
putStrLn has type String -> IO () so you need to convert your list element into a string first.
You can do this with show:
putStrLn $ show $ myLast [1,2,3,4]
data NestedList a = Elem a | List [NestedList a]
flatten :: NestedList a -> [a]
flatten (Elem element) = [element]
flatten (List []) = []
flatten (List (first:rest)) = flatten first ++ flatten (List (rest))
main = print $ flatten $ List []
I wrote the above seen code in haskell. When I execute this with any other parameter, for example
main = print $ flatten $ List [Elem 1, Elem 2]
main = print $ flatten $ Elem 1
It gives
[1, 2]
[1]
respectively.
It fails when I execute it with an empty List.
main = print $ flatten $ List []
Error message
No instance for (Show a0) arising from a use of `print'
The type variable `a0' is ambiguous
Possible fix: add a type signature that fixes these type variable(s)
Note: there are several potential instances:
instance Show Double -- Defined in `GHC.Float'
instance Show Float -- Defined in `GHC.Float'
instance (Integral a, Show a) => Show (GHC.Real.Ratio a)
-- Defined in `GHC.Real'
...plus 23 others
In the expression: print
In the expression: print $ flatten $ List []
In an equation for `main': main = print $ flatten $ List []
Questions
Why does it fail and how can I fix this?
Should I change my NestedList definition to accept an empty List? If so, how do I do that. Its quite confusing.
The list type is polymorphic. Since you don't supply an element, just the empty list constructor [], there's no way to infer what list type this is.
Is it: [] :: [Int]
or [] :: [Maybe (Either String Double)]. Who's to say?
You are. Supply a type annotation to resolve the polymorphism, then GHC can dispatch to the correct show instance.
E.g.
main = print $ flatten $ List ([] :: [Int])
To add to the answers here already, you may object "but what does it matter what type of things my list contains? it doesn't have any of them in it!"
Well, first of all, it is easy to construct situations in which it's unclear whether or not the list is empty, and anyway type checking hates to look at values, it only wants to look at types. This keeps things simpler, because it means that when it comes to deal with values, you can be sure you already know all the types.
Second of all, it actually does matter what kind of list it is, even if it's empty:
ghci> print ([] :: [Int])
[]
ghci> print ([] :: [Char])
""
The problem is the compiler can't know the type of flatten $ List []. Try to figure out the type yourself, you'll see it's [a] for some a, whilst print requires its argument to be an instance of Show, and [a] is an instance of Show if a is an instance of Show. Even though your list is empty, so there's no need for any constraint on a to represent [], there's no way for the compiler to know.
As such, putting an explicit type annotation (for any type for which an instance of Show exists) should work:
main = print $ flatten $ List ([] :: [NestedList Int])
or
main = print $ flatten $ List ([] :: [NestedList ()])
or
main = print fl
where
fl :: [()]
fl = flatten $ List []
[] can be a list of floats, strings, booleans or actually any type at all. Thus, print does not know which instance of show to use.
Do as the error message says and give an explicit type, as in ([] :: [Int]).
I tried writing a simple function that takes an Either type (possibly parameterized by two different types) and does one thing if it gets Left and another thing if it gets Right. the following code,
someFunc :: (Show a, Show b) => Either a b -> IO ()
someFunc (Left x) = print $ "Left " ++ show x
someFunc (Right x) = print $ "Right " ++ show x
main = do
someFunc (Left "Test1")
someFunc (Right "Test2")
However, this gives,
Ambiguous type variable `b0' in the constraint:
(Show b0) arising from a use of `someFunc'
Probable fix: add a type signature that fixes these type variable(s)
In a stmt of a 'do' expression: someFunc (Left "Test1")
and
Ambiguous type variable `a0' in the constraint:
(Show a0) arising from a use of `someFunc'
Probable fix: add a type signature that fixes these type variable(s)
In the expression: someFunc (Right "Test2")
If I understand correctly, when I call the function with Left x, it is complaining because it doesn't know the type of the Right x variant, and vice-versa. However, this branch of the function is not used. Is there a better way to do this?
You can make this work by explicitly specifying the other type:
main = do
someFunc (Left "Test1" :: Either String ())
someFunc (Right "Test2" :: Either () String)
but I agree with x13n that this probably isn't the best way to do whatever you're trying to do. Note that someFunc is functionally identical to
someFunc :: (Show a) => Bool -> a -> IO ()
someFunc False x = print $ "Left " ++ show x
someFunc True x = print $ "Right " ++ show x
because the only information you derive from the structure of the Either is whether it's a Left or Right. This version also doesn't require you to specify a placeholder type when you use it.
This is a good question, because it made me think about why Haskell behaves this way.
class PseudoArbitrary a where
arb :: a
instance PseudoArbitrary Int where
arb = 4
instance PseudoArbitrary Char where
arb = 'd'
instance PseudoArbitrary Bool where
arb = True
reallyDumbFunc :: (PseudoArbitrary a, PseudoArbitrary b) =>
Either a b -> Either a b
reallyDumbFunc (Left x) = Right arb
reallyDumbFunc (Right x) = Left arb
So check this out. I've made a typeclass PseudoArbitrary, where instances of the typeclass provide a (pseudo-)arbitrary element of their type. Now I have a reallyDumbFunction that takes an Either a b, where both a and b have PseudoArbitrary instances, and if a Left was put in, I produce a Right, with a (pseudo-)arbitrary value of type b in it, and vice versa. So now let's play in ghci:
ghci> reallyDumbFunc (Left 'z')
Ambiguous type variable blah blah blah
ghci> reallyDumbFunc (Left 'z' :: Either Char Char)
Right 'd'
ghci> reallyDumbFunc (Left 'z' :: Either Char Int)
Right 4
Whoa! Even though all I changed was the type of the input, it totally changed the type and value of the output! This is why Haskell cannot resolve the ambiguity by itself: because that would require analyzing your function in complicated ways to make sure you are not doing things like reallyDumbFunc.
Well, that depends heavily on what you are trying to do, doesn't it? As you already found out, in order to use Left constructor, you need to know the type it constructs. And full type requires information on both a and b.
Better way to achieve polymorphism in Haskell is to use type classes. You can easily provide different implementations of "methods" for different instances.
Good comparison of both object-orientation and type classes concepts can be found here.