Haskell/YAHT/Language advanced/Solutions

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Yet Another Haskell Tutorial
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Language Basics (Solutions)
Type Basics (Solutions)
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Modules (Solutions)
Advanced Language (Solutions)
Advanced Types (Solutions)
Monads (Solutions)
Advanced IO

Sections and Infix OperatorsEdit

Local DeclarationsEdit

Partial ApplicationEdit

Function func3 cannot be converted into point-free style. The others look something like:

func1 x = map (*x)

func2 f g = filter f . map g

func4 = map (+2) . filter (`elem` [1..10]) . (5:)

func5 = flip foldr 0 . flip . curry 

You might have been tempted to try to write func2 as filter f . map, trying to eta-reduce off the g. In this case, this isn't possible. This is because the function composition operator (.) has type (b -> c) -> (a -> b) -> (a -> c). In this case, we're trying to use map as the second argument. But map takes two arguments, while (.) expects a function which takes only one.

Pattern MatchingEdit


Instance DeclarationsEdit

The Eq ClassEdit

The Show ClassEdit

Other Important ClassesEdit

The Ord ClassEdit

The Enum ClassEdit

The Num ClassEdit

The Read ClassEdit

Class ContextsEdit

Deriving ClassesEdit

Datatypes RevisitedEdit

Named FieldsEdit

More ListsEdit

Standard List FunctionsEdit

List ComprehensionsEdit


Finite MapsEdit


The Final Word on ListsEdit

We can start out with a recursive definition:

and [] = True
and (x:xs) = x && and xs

From here, we can clearly rewrite this as:

and = foldr (&&) True

We can write this recursively as:

concatMap f [] = []
concatMap f (x:xs) = f x ++ concatMap f xs

This hints that we can write this as:

concatMap f = foldr (\a b -> f a ++ b) []

Now, we can do point elimination to get:

     foldr (\a b -> f a ++ b) []
==>  foldr (\a b -> (++) (f a) b) []
==>  foldr (\a -> (++) (f a)) []
==>  foldr (\a -> ((++) . f) a) []
==>  foldr ((++) . f) []
Last modified on 26 June 2007, at 13:28