I'm trying to figure out what is the best practice to write the Haskell equivalent of assert(0). I know that type safety dictates that an integer must be returned from scanList, however I wonder if there's a better way than what I wrote. Is there any way to avoid the arbitrary number 923 that is just stuck there?
module Main (main) where
import Control.Exception (assert)
l = [
Left "1",
Left "1",
Right 1,
Right 1,
Left "9"]
scanList :: [ Either String Int ] -> Int
scanList [ ] = 0
scanList (x:xs) = case x of
Right i -> i + scanList xs
Left s ->
if read s < 8
then read s + scanList xs
else assert False $ 923
main = do
print $ scanList l
From the documentation of assert:
If the first argument evaluates to True, then the result is the second argument. Otherwise an AssertionFailed exception is raised, containing a String with the source file and line number of the call to assert.
So instead of giving False as first argument you could actually check your if condition there:
scanList (x:xs) = case x of
Right i -> i + scanList xs
Left s ->
assert (read s < 8) (read s + scanList xs)
The more idiomatic Haskell design would be to keep pure functions total. When using assert, you're throwing an exception, which makes the function partial. This means that you can no longer trust the type of the function. It claims to have the type [Either String Int] -> Int, but will fail with an exception at run-time under various conditions.
A total function would either stay within the Either monad, or could, alternatively, translate to Maybe:
import Text.Read
scanList :: (Num a, Read a, Ord a) => [Either String a] -> Maybe a
scanList [] = Just 0
scanList (x:xs) =
case x of
Right i -> fmap (+ i) $ scanList xs
Left s ->
case readMaybe s of
Just i -> if i < 8 then fmap (+ i) $ scanList xs else Nothing
Nothing -> Nothing
You could simplify the code quite a bit, but I chose to keep it structurally as close to the OP as possible.
With a type like [Either String a] -> Maybe a, any caller knows that they have to handle both Just and Nothing cases, without having to resort to reading the code or documentation of the function in question.
Related
I'd like to create a list data structure that can zipWith that has a better behavior with self reference. This is for an esoteric language that will rely on self reference and laziness to be Turing complete using only values (no user functions). I've already created it, called Atlas but it has many built ins, I'd like to reduce that and be able to compile/interpret in Haskell.
The issue is that zipWith checks if either list is empty and returns empty. But in the case that this answer depends on the result of zipWith then it will loop infinitely. Essentially I'd like it to detect this case and have faith that the list won't be empty. Here is an example using DList
import Data.DList
import Data.List (uncons)
zipDL :: (a->b->c) -> DList a -> DList b -> DList c
zipDL f a b = fromList $ zipL f (toList a) (toList b)
zipL :: (a->b->c) -> [a] -> [b] -> [c]
zipL _ [] _ = []
zipL _ _ [] = []
zipL f ~(a:as) ~(b:bs) = f a b : zipL f as bs
a = fromList [5,6,7]
main=print $ dh where
d = zipDL (+) a $ snoc (fromList dt) 0
~(Just (dh,dt)) = uncons $ toList d
This code would sum the list 5,6,7 except for the issue. It can be fixed by removing zipL _ _ [] = [] because then it assumes that the result won't be empty and then it in fact turns out not to be empty. But this is a bad solution because we can't always assume that it is the second list that could have the self reference.
Another way of explaining it is if we talk about the sizes of these list.
The size of zip a b = min (size a) (size b)
So in this example: size d = min (size a) (size d-1+1)
But there in lies the problem, if the size of d is 0, then the size of d = 0, but if size of d is 1 the size is 1, however once the size of d is said to be greater than size of a, then the size would be a, which is a contradiction. But any size 0-a works which means it is undefined.
Essentially I want to detect this case and make the size of d = a.
So far the only thing I have figured out is to make all lists lists of Maybe, and terminate lists with a Nothing value. Then in the application of the zipWith binary function return Nothing if either value is Nothing. You can then take out both of the [] checks in zip, because you can think of all lists as being infinite. Finally to make the summation example work, instead of doing a snoc, do a map, and replace any Nothing value with the snoc value. This works because when checking the second list for Nothing, it can lazily return true, since no value of the second list can be nothing.
Here is that code:
import Data.Maybe
data L a = L (Maybe a) (L a)
nil :: L a
nil = L Nothing nil
fromL :: [a] -> L a
fromL [] = nil
fromL (x:xs) = L (Just x) (fromL xs)
binOpMaybe :: (a->b->c) -> Maybe a -> Maybe b -> Maybe c
binOpMaybe f Nothing _ = Nothing
binOpMaybe f _ Nothing = Nothing
binOpMaybe f (Just a) (Just b) = Just (f a b)
zip2W :: (a->b->c) -> L a -> L b -> L c
zip2W f ~(L a as) ~(L b bs) = L (binOpMaybe f a b) (zip2W f as bs)
unconsL :: L a -> (Maybe a, Maybe (L a))
unconsL ~(L a as) = (a, Just as)
mapOr :: a -> L a -> L a
mapOr v ~(L a as) = L (Just $ fromMaybe v a) $ mapOr v as
main=print $ h
where
a = fromL [4,5,6]
b = zip2W (+) a (mapOr 0 (fromJust t))
(h,t) = unconsL $ b
The downside to this approach is it needs this other operator to map with Just . fromMaybe initialvalue. This is a less intuitive operator than ++. And without it the language could be built entirely on ++ uncons and (:[]) which would be pretty neat.
The other thing I've figured out is in the current ruby implementation to throw an error when a value depends on itself, and catch it in the empty list detection. But this is vary hacky and not entirely sound, although it does work for cases like this. I don't think this can work in Haskell since I don't think you can detect self dependence?
Sorry for the long description and the very odd use case. I've spent tons of time thinking about this, but haven't solved it yet and can't explain it any more succinctly! Not expecting an answer but figured it is worth a shot, thanks for considering.
EDIT:
After seeing it framed as a greatest fixed point question, it seems like a poor question because there is no efficient general solution to such a problem. For example, suppose the code was b = zipWith (+) a (if length b < 1 then [1] else []).
For my purposes it could still be nice to handle some cases correctly - the example provided does have a solution. So I could reframe the question as: when can we find the greatest fixed point efficiently and what is that fixed point? But I believe there is no simple answer to such a question, and so it would be a poor basis for a programming language to rely on ad hoc rules.
Sounds like you want a greatest fixed point. I'm not sure I've seen this done before, but maybe it's possible to make a sensible type class for types that support those.
class GF a where gfix :: (a -> a) -> a
instance GF a => GF [a] where
gfix f = case (f (repeat undefined), f []) of
(_:_, _) -> b:bs where
b = gfix (\a' -> head (f (a':bs)))
bs = gfix (\as' -> tail (f (b:as')))
([], []) -> []
_ -> error "no fixed point greater than bottom exists"
-- use the usual least fixed point. this ain't quite right, but
-- it works for this example, and maybe it's Good Enough
instance GF Int where gfix f = let x = f x in x
Try it out in ghci:
> gfix (\xs -> zipWith (+) [5,6,7] (tail xs ++ [0])) :: [Int]
[18,13,7]
This implementation isn't particularly efficient; e.g. replacing [5,6,7] with [1..n] results in a runtime that's quadratic in n. Perhaps with some cleverness that can be improved, but it's not immediately obvious to me how that would go.
I have an answer for this specific case, not general.
appendRepeat :: a -> [a] -> [a]
appendRepeat v a = h : appendRepeat v t
where
~(h,t) =
if null a
then (v,[])
else (head a,tail a)
a = [4,5,6]
main=print $ head b
where
b = zipWith (+) a $ appendRepeat 0 (tail b)
appendRepeat adds a an infinite list of a repeated value to the end of a list. But the key thing about it is it doesn't check if list is empty or not when deciding that it is returning a non empty list where the tail is a recursive call. This way laziness never ends up in an infinite loop checking the zipWith _ [] case.
So this code works, and for the purposes of the original question, it can be used to convert the language to just using 2 simple functions (++ and :[]). But the interpreter would need to do some static analysis for appending a repeated value and replace it to using this special appendRepeat function (which can easily be done in Atlas). It seems hacky to only make this one implementation switcharoo, but that is all that is needed.
I'm writing a function that gets the index of the first even number from a list. The list I get may or may not contain even numbers, and I'd like to return -1 if there are no even numbers in the list. The list can be infinite.
I wrote this
posicPrimerPar'' :: [Int] -> Int
posicPrimerPar'' a = fromJust (elemIndex (head (filter (even) a)) a)
I could do something like:
posicPrimerPar' :: [Int] -> Int
posicPrimerPar' a = case length evens of
0 -> -1;
n -> fromJust elemIndex (head evens) a
where evens = filter (even) a
But as you can see, this is not the most efficient way of doing it. A list [1..100000] contains a lot of even numbers, and I just need the first one. I need Haskell's laziness, so I need to ask for the head right there, but head throws an empty list exception when the list is empty (i.e. there are no even numbers in the list). I cannot find the Haskell equivalent of Python's try: ... except: .... All I could find regarding exceptions were IO related. What I need is except Prelude.head = -1 or something like that.
Haskell is lazy, so evens will not be fully evaluated. The problematic part is the length evens which is not necessary. You can check with null :: Foldable f => f a -> Bool, or with pattern matching. For example:
import Data.List(findIndex)
posicPrimerPar' :: [Int] -> Maybe Int
posicPrimerPar' [] = Nothing
posicPrimerPar' xs = findIndex even xs
for findIndex :: (a -> Bool) -> [a] -> Maybe Int, you however do not need to take into account the empty list, since it already considers this.
or we can return -1 in case there is no such item:
import Data.List(findIndex)
import Data.Maybe(fromMaybe)
posicPrimerPar' :: [Int] -> Int
posicPrimerPar' = fromMaybe (-1) . findIndex even
I'm still new to Haskell. Whats the best way to distinguish between errors?
Currently I'm using the maybe monad but it can only 'represent one state of error'.
The following code snippet will put my question into context.
pulOh :: Seq -- input X O sequence
-> Int -- split point real index
-> Maybe Seq
pulOh xs n =
case (\(f,l)->(tlTrn f, hdTrn l)) (splSq xs n) of -- split and process at index
(Nothing, _) -> Nothing -- first failed
(_, Nothing) -> Nothing -- last failed
(Just f,Just l) -> Just (f ++ l) -- both parts passed
I would like the result to be able to distinguish if the call fails for fst or snd. Short-circuiting the case of both failing to fst fail.
Use Either. It is basically the same as Maybe with a parameterised Nothing constructor, or in other words, Maybe a is isomorphic to Either () a. By replacing the () “unit error” with a custom error-tag type, you can make different failure cases different.
pulOh :: Seq -> Int -> Either String Seq
pulOh xs n = case tlTrn *** hdTrn $ splSq xs n of
(Nothing, _) -> Left "first failed"
(_, Nothing) -> Left "last failed"
(Just f,Just l) -> Right $ f ++ l
(I took the liberty of replacing that lambda with a *** “parallel pipeline”)
I recently started learning Haskell and I'm trying to rewrite something I did for an interview in python in Haskell. I'm trying to convert a string from camel case to underscore separated ("myVariableName" -> "my_variable_name"), and also throw an error if the first character is upper case.
Here's what I have:
import qualified Data.Char as Char
translate_java :: String -> String
translate_java xs = translate_helper $ enumerate xs
where
translate_helper [] = []
translate_helper ((a, num):xs)
| num == 1 and Char.isUpper a = error "cannot start with upper"
| Char.isUpper a = '_' : Char.toLower a : translate_helper xs
| otherwise = a : translate_helper xs
enumerate :: (Num b, Enum b) => [a] -> [(a,b)]
enumerate xs = zip xs [1..]
I realize It's pretty likely I'm going about this in a weird way, and I'd love advice about better ways to implement this, but I'd like to get this to compile as well. Here's the error I'm getting now:
Prelude> :r
[1 of 1] Compiling Main ( translate.hs, interpreted )
translate.hs:4:20:
No instance for (Num
(([Bool] -> Bool) -> (Char -> Bool) -> Char -> t))
arising from a use of `translate_helper' at translate.hs:4:20-35
Possible fix:
add an instance declaration for
(Num (([Bool] -> Bool) -> (Char -> Bool) -> Char -> t))
In the first argument of `($)', namely `translate_helper'
In the expression: translate_helper $ enumerate xs
In the definition of `translate_java':
translate_java xs
= translate_helper $ enumerate xs
where
translate_helper [] = []
translate_helper ((a, num) : xs)
| num == 1 and Char.isUpper a
= error "cannot start with upper
"
| Char.isUpper a
= '_' : Char.toLower a : transla
te_helper xs
| otherwise = a : translate_help
er xs
Failed, modules loaded: none.
Any explanation of what's going on here would be great. I really don't understand where "(Num (([Bool] -> Bool) -> (Char -> Bool) -> Char -> t))" is coming from. I'd think the type declaration for translate_helper would be something like [(a,b)] -> [a]?
You have to replace and by &&. The first one is a function (prefix) that receives a list of boolean values and calculates an and of them all. The second one is a true logical and. The error message is a little bit confusing though. Whenever I get such a strange error message, I usually start to annotate my code with type signatures. Then the compiler is able to give you a more detailed description of what went wrong.
Others have mentioned that you should use (&&) instead of and, so I'll answer your other question: no, I don't think you're going about this in a weird way.
But... I do think it can be even more elegant!
translate_java (x:xs) | isUpper x = error "cannot start with an upper"
translate_java xs = concatMap translate xs where
translate x = ['_' | isUpper x] ++ [toLower x]
There's a few interesting things going on here:
The special case is checked straight away. Don't wait until you're recursing to do this!
The concatMap function is really handy in a lot of cases. It's just a map followed by a concat. If I were writing this myself, I'd probably use xs >>= translate instead.
That ['_' | isUpper x] is a list comprehension; this is a cute idiom for making a list with either 0 or 1 elements in it, depending on whether a predicate holds.
Other than that, the code should be fairly self-explanatory.
The problem is this:
| num == 1 and Char.isUpper a = ...
and is not an infix operator; rather it is a function:
and :: [Bool] -> Bool
So it is interpreting 1 and Char.isUpper a as applying three arguments to the "function" 1. Use && instead.
The error message comes from the way numerals are interpreted. A numeral, say, 1 is actually polymorphic; the specific type it gets depends on the type that is needed. That's why you can say x+1 and it will work whether x is an integer or a double or whatever. So the compiler inferred that the type of 1 needs to be a three-argument function, and then tried to find a numeric type matching that so it could convert 1 into that type (and, naturally, failed).
Here's my solution. It's not as masterful as the answer Daniel Wagner gave using concatMap and the list comprehension, but it's perhaps easier to understand for the beginner.
conv :: String -> String
conv [] = []
conv s#(x:xs) = if Char.isUpper x
then error "First character cannot be uppercase"
else change s
change :: String -> String
change [] = []
change (x:xs) = if Char.isUpper x
then '_' : Char.toLower x : change xs
else x : change xs
The function conv really just checks your criterion that the first character must not be uppercase, and if it isn't it hands over the string to the function change, which does the work. It goes through all the characters one by one, building a list, and if the character is uppercase, it adds an underscore followed by the lowercase version of the character, otherwise if the character is already lowercase it just adds it as it is.
I'm trying to understand how Haskell list comprehensions work "under the hood" in regards to pattern matching. The following ghci output illustrates my point:
Prelude> let myList = [Just 1, Just 2, Nothing, Just 3]
Prelude> let xs = [x | Just x <- myList]
Prelude> xs
[1,2,3]
Prelude>
As you can see, it is able to skip the "Nothing" and select only the "Just" values. I understand that List is a monad, defined as (source from Real World Haskell, ch. 14):
instance Monad [] where
return x = [x]
xs >>= f = concat (map f xs)
xs >> f = concat (map (\_ -> f) xs)
fail _ = []
Therefore, a list comprehension basically builds a singleton list for every element selected in the list comprehension and concatenates them. If a pattern match fails at some step, the result of the "fail" function is used instead. In other words, the "Just x" pattern doesn't match so [] is used as a placeholder until 'concat' is called. That explains why the "Nothing" appears to be skipped.
What I don't understand is, how does Haskell know to call the "fail" function? Is it "compiler magic", or functionality that you can write yourself in Haskell? Is it possible to write the following "select" function to work the same way as a list comprehension?
select :: (a -> b) -> [a] -> [b]
select (Just x -> x) myList -- how to prevent the lambda from raising an error?
[1,2,3]
While implemenatations of Haskell might not do it directly like this internally, it is helpful to think about it this way :)
[x | Just x <- myList]
... becomes:
do
Just x <- myList
return x
... which is:
myList >>= \(Just x) -> return x
As to your question:
What I don't understand is, how does Haskell know to call the "fail" function?
In do-notation, if a pattern binding fails (i.e. the Just x), then the fail method is called. For the above example, it would look something like this:
myList >>= \temp -> case temp of
(Just x) -> return x
_ -> fail "..."
So, every time you have a pattern-match in a monadic context that may fail, Haskell inserts a call to fail. Try it out with IO:
main = do
(1,x) <- return (0,2)
print x -- x would be 2, but the pattern match fails
The rule for desugaring a list comprehension requires an expression of the form [ e | p <- l ] (where e is an expression, p a pattern, and l a list expression) behave like
let ok p = [e]
ok _ = []
in concatMap ok l
Previous versions of Haskell had monad comprehensions, which were removed from the language because they were hard to read and redundant with the do-notation. (List comprehensions are redundant, too, but they aren't so hard to read.) I think desugaring [ e | p <- l ] as a monad (or, to be precise, as a monad with zero) would yield something like
let ok p = return e
ok _ = mzero
in l >>= ok
where mzero is from the MonadPlus class. This is very close to
do { p <- l; return e }
which desugars to
let ok p = return e
ok _ = fail "..."
in l >>= ok
When we take the List Monad, we have
return e = [e]
mzero = fail _ = []
(>>=) = flip concatMap
I.e., the 3 approaches (list comprehensions, monad comprehensions, do expressions) are equivalent for lists.
I don't think the list comprehension syntax has much to do with the fact that List ([]), or Maybe for that matter, happens to be an instance of the Monad type class.
List comprehensions are indeed compiler magic or syntax sugar, but that's possible because the compiler knows the structure of the [] data type.
Here's what the list comprehension is compiled to: (Well, I think, I didn't actually check it against the GHC)
xs = let f = \xs -> case xs of
Just x -> [x]
_ -> []
in concatMap f myList
As you can see, the compiler doesn't have to call the fail function, it can simply inline a empty list, because it knows what a list is.
Interestingly, this fact that the list comprehensions syntax 'skips' pattern match failures is used in some libraries to do generic programming. See the example in the Uniplate library.
Edit: Oh, and to answer your question, you can't call your select function with the lambda you gave it. It will indeed fail on a pattern match failure if you call it with an Nothing value.
You could pass it the f function from the code above, but than select would have the type:
select :: (a -> [b]) -> [a] -> [b]
which is perfectly fine, you can use the concatMap function internally :-)
Also, that new select now has the type of the monadic bind operator for lists (with its arguments flipped):
(>>=) :: [a] -> (a -> [b]) -> [b]
xs >>= f = concatMap f xs -- 'or as you said: concat (map f xs)