1

:t interact

interact :: (String -> String) -> IO ()

This is treated as part of the program

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words "foo bar baz"

["foo","bar","baz"]

Convert strings to numbers

You must also specify the type.

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(read "5") :: Int

5

Use map

It applies a function to a list and results in a new list.

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:t map

map :: (a -> b) -> [a] -> [b]

The result should be list of integer

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(map read $ words "1 2") :: [Int]

[1,2]

Without specifying the result, read will not work.

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(map read $ words "1 2")

[*** Exception: Prelude.read: no parse

map and read

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:t map read

map read :: Read b => [String] -> [b]

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:t sum . map read

sum . map read :: (Num c, Read c) => [String] -> c

sum

I guess it’s hard to make sense of types when I don’t use parentheses.

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map read $ words "1 2" :: [Int]

[1,2]

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sum $ map read $ words "1 2" :: Int

3

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(sum $ map read $ words "1 2") :: Int

3

Since sum already specifies a number, we don’t have to specify a return type.

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sum $ map read $ words "1 2"

3

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show $ sum $ map read $ words "1 2"

"3"

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1 2

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main = interact $ show . sum . map read . words

3