# Difference between revisions of "99 questions/Solutions/8"

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< 99 questions | Solutions

(Changed "haskell" tag to "pre", to display the error message properly) |
(Removed the remark about the monomorphism restriction, as the restriction is removed in haskell 2010) |
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We simply group equal values together (using Data.List.group), then take the head of each. |
We simply group equal values together (using Data.List.group), then take the head of each. |
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− | Note that (with GHC) we must give an explicit type to ''compress'' otherwise we get: |
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− | |||

− | <pre> |
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− | Ambiguous type variable `a' in the constraint: |
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− | `Eq a' |
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− | arising from use of `group' |
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− | Possible cause: the monomorphism restriction applied to the following: |
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− | compress :: [a] -> [a] |
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− | Probable fix: give these definition(s) an explicit type signature |
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− | or use -fno-monomorphism-restriction |
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− | </pre> |
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− | |||

− | We can circumvent the monomorphism restriction by writing ''compress'' this way (See: section 4.5.4 of [http://haskell.org/onlinereport the report]): |
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− | <haskell>compress xs = map head $ group xs</haskell> |
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An alternative solution is |
An alternative solution is |
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| otherwise = x : compress ys |
| otherwise = x : compress ys |
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compress ys = ys |
compress ys = ys |
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− | </haskell> |
+ | </haskell><br> |

A variation of the above using <hask>foldr</hask> (note that GHC erases the <hask>Maybe</hask>s, producing efficient code): |
A variation of the above using <hask>foldr</hask> (note that GHC erases the <hask>Maybe</hask>s, producing efficient code): |
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-- [2,1] - must be [1,2,1] |
-- [2,1] - must be [1,2,1] |
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</haskell> |
</haskell> |
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− | |||

## Revision as of 13:44, 15 February 2015

(**) Eliminate consecutive duplicates of list elements.

```
compress :: Eq a => [a] -> [a]
compress = map head . group
```

We simply group equal values together (using Data.List.group), then take the head of each.

An alternative solution is

```
compress (x:ys@(y:_))
| x == y = compress ys
| otherwise = x : compress ys
compress ys = ys
```

A variation of the above using `foldr`

(note that GHC erases the `Maybe`

s, producing efficient code):

```
compress xs = foldr f (const []) xs Nothing
where
f x r a@(Just q) | x == q = r a
f x r _ = x : r (Just x)
```

Another possibility using foldr (this one is not so efficient, because it pushes the whole input onto the "stack" before doing anything else):

```
compress :: (Eq a) => [a] -> [a]
compress = foldr skipDups []
where skipDups x [] = [x]
skipDups x acc
| x == head acc = acc
| otherwise = x : acc
```

A very simple approach:

```
compress [] = []
compress (x:xs) = x : (compress $ dropWhile (== x) xs)
```

Another approach, using foldr

```
compress :: Eq a => [a] -> [a]
compress x = foldr (\a b -> if a == (head b) then b else a:b) [last x] x
```

Wrong solution using foldr

```
compress :: Eq a => [a] -> [a]
compress xs = foldr (\x acc -> if x `elem` acc then acc else x:acc) [] xs
-- Main> compress [1, 1, 1, 2, 2, 1, 1]
-- [2,1] - must be [1,2,1]
```

and using foldl

```
compress :: (Eq a) => [a] -> [a]
compress x = foldl (\a b -> if (last a) == b then a else a ++ [b]) [head x] x
compress' x = reverse $ foldl (\a b -> if (head a) == b then a else b:a) [head x] x
```

A crazy variation that acts as a good transformer for fold/build fusion

```
{-# INLINE compress #-}
compress :: Eq a => [a] -> [a]
compress xs = build (\c n ->
let
f x r a@(Just q) | x == q = r a
f x r _ = x `c` r (Just x)
in
foldr f (const n) xs Nothing)
```