Chapter 1 Tasks #557
Chapter 1 Tasks #557
Conversation
I reinstalled ghc and it helped. Fixed some issues in last commit. |
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@vrom911 andM (Just False) Nothing Should not it be Nothing instead of Just False? Could you review 3 chapters done for now, please? :) |
| @@ -429,6 +429,7 @@ task is to specify the type of this function. | |||
| 49 | |||
| -} | |||
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| squareSum :: Num a => a -> a -> a | |||
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We explain Num in the following chapters, so you are one step ahead 😸
In here Int -> Int -> Int would also work, as a more specific type 🙂
| | c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u' = True | ||
| | otherwise = False |
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Yes, this is right!
As || operator already returns the Bool, you can also skip the cases and use just it:
| | c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u' = True | |
| | otherwise = False | |
| = c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u' |
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Yes, this is right!
As
||operator already returns theBool, you can also skip the cases and use just it:
Oh, yeah :) I've just forgot it for a moment))
| a = an `mod` 10 | ||
| b = (an `div` 10) `mod` 10 |
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That is a wonderful solution! 👏🏼 You correctly noticed that it is the div and mod, cool 😎
One hint to make your solution even shorter: you can see that you use both:
mod m 10
div m 10The standard library has the divMod function, that actually combines inside both div and mod. And this is exactly what you use!.
So you could write it this way:
(x, y) = divMod m 10You can see how we could pattern match on the pair 🙂
| takeEven [] = [] | ||
| takeEven [x] = [x] | ||
| takeEven (x:_:xs) = x : takeEven xs |
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That's great!
Note, that in the first two cases you return the list as it is. In this case, you could also right the function with just two cases, by switching them around:
takeEven (x:_:xs) = x : takeEven xs
takeEven xs = xsThough, I think that your solution is more explicit, therefore, more readable! 👏🏼
| @@ -728,7 +760,7 @@ value of the element itself | |||
| 🕯 HINT: Use combination of 'map' and 'replicate' | |||
| -} | |||
| smartReplicate :: [Int] -> [Int] | |||
| smartReplicate l = error "smartReplicate: Not implemented!" | |||
| smartReplicate l = concat (map (\x -> replicate x x) l) | |||
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Great! 👍🏼
If you want to write this function even more elegantly, you can use the standard function concatMap which is a combination of concat and map as the name suggests 🙂
After that, you could notice, that you would be able to eta-reduce on the last argument :)
| rotate :: Int -> [a] -> [a] | ||
| rotate n l | ||
| | n < 0 = [] | ||
| | otherwise = take (length l) (drop n (cycle l)) |
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Awesome!
You can optimise it a little bit by using mod n (length l) as in this case you won't overgo if n is too huge 🙂
| rewind l = rewind' [] l where | ||
| rewind' reversed [] = reversed | ||
| rewind' reversed (x:xs) = rewind' (x : reversed) xs |
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Fantastic solution 🎉
Nothe, that there is no need to pattern match on the empty list in the rewind itself as you check this case in rewind' anyway, so you can get rid of this case 🙂
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| (<*>) :: Secret e (a -> b) -> Secret e a -> Secret e b | ||
| (<*>) = error "(<*>) Secret: Not implemented!" | ||
| (Trap f) <*> _ = Trap f | ||
| _ <*> (Trap x) = Trap x |
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This case is unnecessary and should be covered by the next case 👍🏼
| concatList Empty xs = xs | ||
| concatList xs Empty = xs | ||
| concatList xs (Cons y ys) = concatList (Cons y xs) ys |
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You can put this function on the global level as it is used in two places already 👌🏼
| @@ -628,7 +661,7 @@ Can you implement a monad version of AND, polymorphic over any monad? | |||
| 🕯 HINT: Use "(>>=)", "pure" and anonymous function | |||
| -} | |||
| andM :: (Monad m) => m Bool -> m Bool -> m Bool | |||
| andM = error "andM: Not implemented!" | |||
| andM mx my = mx >>= \x -> my >>= \y -> pure $ x && y | |||
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Regarding your question, in the test case Just False is there on purpose.
The ordinary && if lazy on the second argument. Meaning that if it meets False as the first argument, there is no need to calculate another arg. In andM we would like to keep the same behaviour, so the
Just False `andM` _
should be Just False 🙂
If you don't use the >>= operator, then the Monad constraint is too strong, and it is enough to specify only Applicative. However, with Applicative you are not able to implement a lazier version of andM.
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For my code this test fails. How can I change code to pass this test case? Don't understand yet.
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Sure, no problem! All you need to do is to first check on the first value and stop lazily in case it is False. The code could look. like this:
andM ma mb = ma >>= \a -> if a then mb else pure FalseThere was a problem hiding this comment.
I see. Thank you for your help and review :)
Could you suggest what can be done next to improve Haskell skills?
Solutions for Chapter 1
cc @vrom911
Hi. I've solved tasks in chapter 1, but can't run tests for some reasons: (could you help with that?)
make test-chapter1
cabal test doctest-chapter1 --enable-tests --test-show-details=direct
Resolving dependencies...
Error: cabal.exe: Could not resolve dependencies:
[__0] trying: learn4haskell-0.0.0.0 (user goal)
[__1] next goal: base (dependency of learn4haskell)
[__1] rejecting: base-4.17.0.0/installed-4.17.0.0 (conflict: learn4haskell =>
base>=4.14.0.0 && <4.17)
[__1] skipping: base-4.18.0.0, base-4.17.0.0 (has the same characteristics
that caused the previous version to fail: excluded by constraint '>=4.14.0.0
&& <4.17' from 'learn4haskell')
[__1] rejecting: base-4.16.4.0, base-4.16.3.0, base-4.16.2.0, base-4.16.1.0,
base-4.16.0.0, base-4.15.1.0, base-4.15.0.0, base-4.14.3.0, base-4.14.2.0,
base-4.14.1.0, base-4.14.0.0, base-4.13.0.0, base-4.12.0.0, base-4.11.1.0,
base-4.11.0.0, base-4.10.1.0, base-4.10.0.0, base-4.9.1.0, base-4.9.0.0,
base-4.8.2.0, base-4.8.1.0, base-4.8.0.0, base-4.7.0.2, base-4.7.0.1,
base-4.7.0.0, base-4.6.0.1, base-4.6.0.0, base-4.5.1.0, base-4.5.0.0,
base-4.4.1.0, base-4.4.0.0, base-4.3.1.0, base-4.3.0.0, base-4.2.0.2,
base-4.2.0.1, base-4.2.0.0, base-4.1.0.0, base-4.0.0.0, base-3.0.3.2,
base-3.0.3.1 (constraint from non-upgradeable package requires installed
instance)
[__1] fail (backjumping, conflict set: base, learn4haskell)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: base, learn4haskell
make: *** [Makefile:11: test-chapter1] Error 1