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Today's Topics:
1. Iterate in terms of scanl (Atrudyjane)
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Message: 1
Date: Sun, 23 Oct 2016 21:37:14 -0400
From: Atrudyjane <[email protected]>
To: "[email protected]" <[email protected]>
Subject: [Haskell-beginners] Iterate in terms of scanl
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Hello All,
Sorry about my previous post, email was hosed. I was working on a homework
problem where the task is to write the iterate function in terms of scanl. Came
up with this:
myIterate f x = scanl (const.f) x (repeat x)
I went looking around for other solutions to check my work and found the
following solution on the Haskell Wiki:
iterate f x = scanl f x (repeat x)
myIterate seems to work checked against the Prelude iterate but I don't know if
it's a good solution or not, because the iterate solution on the Haskell Wiki
throws a type error. Types of Prelude iterate and scanl are different, so I can
see why, or am I missing something? Here is some REPL output:
Prelude iterate:
λ> take 10 (iterate (+1) 1)
[1,2,3,4,5,6,7,8,9,10]
myIterate:
λ> take 10 (myIterate (+1) 1)
[1,2,3,4,5,6,7,8,9,10]
Haskell Wiki solution:
λ> take 10 (iterate' (+1) 1)
error:
• Occurs check: cannot construct the infinite type: a ~ a -> a
Expected type: a -> a -> a
Actual type: (a -> a) -> a -> a
• In the first argument of ‘iterate'’, namely ‘(+ 1)’
In the second argument of ‘take’, namely ‘(iterate' (+ 1) 1)’
In the expression: take 10 (iterate' (+ 1) 1)
• Relevant bindings include it :: [a] (bound at <interactive>:63:1)
It will work if you pass it a function that takes two parameters though:
λ> take 10 (iterate' (+) 1)
[1,2,3,4,5,6,7,8,9,10]
Any thoughts would be much appreciated!
Andrea
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