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Today's Topics:
1. Re: Hoogle q5.8: write your own findindices (trent shipley)
----------------------------------------------------------------------
Message: 1
Date: Thu, 18 Jan 2018 07:58:08 +0000
From: trent shipley <trent.ship...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Hoogle q5.8: write your own
findindices
Message-ID:
<caeflybjgz7l_n4ss4s7rh4hcgqydf9pn0jci0nuhbbj30v_...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
{-
8. Redefine the function positions using the function find.
-}
positions :: Eq a => a -> [a] -> [Int]
positions x xs = [i | (x', i) <- zip xs [0..], x == x']
{-
Hutton, Graham. Programming in Haskell (Kindle Locations 1640-1642).
Cambridge University Press. Kindle Edition.
-}
find :: Eq a => a -> [(a,b)] -> [b]
find k t = [v | (k',v) <- t, k == k']
{-
Hutton, Graham. Programming in Haskell (p. 49). Cambridge University Press.
Kindle Edition.
-}
positions' :: Eq a => a -> [a] -> [Int]
positions' x xs = find x (zip xs [0..])
-------------------------------------------------------------------
Having read the chapter a week ago, I had forgotten that Hutton had
previously defined a function "find" for tuples (see above). After trying a
search of the Kindle book the solution was trivial.
Trent.
>
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