Thanks, Theodore. As simple as that! Best regards, Peter On 17 December 2017 at 15:40, Theodore Lief Gannon wrote:

Because: (!!) :: [a] -> Int -> a

On Dec 17, 2017 5:50 AM, "Peter Normington" wrote:

...

Why do I get an error in the following example:

memoized_fib :: Integer -> Integer memoized_fib = (map fib [0 ..] !!) where fib 1 = 1 fib 2 = 1 fib n = memoized_fib (n-2) + memoized_fib (n-1)

but not in:

memoized_fib :: Int -> Integer memoized_fib = (map fib [0 ..] !!) where fib 1 = 1 fib 2 = 1 fib n = memoized_fib (n-2) + memoized_fib (n-1) ?

The error is reported by GHCi as:

error: • Couldn't match type ‘Integer’ with ‘Int’ Expected type: Integer -> Integer Actual type: Int -> Integer • In the expression: (map fib [0 .. ] !!) In an equation for ‘memoized_fib’: memoized_fib = (map fib [0 .. ] !!) where fib 1 = 1 fib 2 = 1 fib n = memoized_fib (n - 2) + memoized_fib (n - 1) | 2 | memoized_fib = (map fib [0 ..] !!) | ^^^^^^^^^^^^^^^^^ Failed, 0 modules loaded.

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