Re: [Haskell] Function to replace given element in list

19 Jul 2016

      Hi Niely,

everything after the "in" in your "let in" is one expression. In other
words all of this:
                        init x
                        x ++ newNmr
                        x ++ y

is being read as one line.. so

init x x ++ newNmr x ++ y

which is why the compiler is complaining that you are applying init to two
values (init x x).

I think what you want is actually something more like

(init x) ++ [newNmr] ++ y

assuming newNmr is a number and not already a list.

I'm not entirely sure what you were trying to do there but you seem a bit
confused.. "init x" on its own doesn't actually do anything; it computes a
value but you aren't assigning it to anything here.

If you wanted to keep those exact expressions you could instead do

let x0 = init x
     x1 = x0 ++ newNmr
in x1 ++ y

and x1 ++ y would be the thing that actually gets return.

In any case I am certainly no expert so hopefully someone else can explain
this better than I can.

On Tue, Jul 19, 2016 at 1:08 PM, Niely Boyken <niely.b0yk3n@gmail.com>
wrote:
...
Hi
I'm trying to make a custom function to replace a given element in a list.
Code:
    let i = elemIndex toReplace lst in
case i of
            Just i ->
                let z = splitAt i lst
                    x = fst z
                    y = (snd z)
                    in
                        init x
                        x ++ newNmr
                        x ++ y
Nothing -> [5]
Error:
...
1)
 * Couldn't match expected type `[a] -> [a]' with actual type `[a]'
    * The function `init' is applied to two arguments,
      but its type `[a] -> [a]' has only one
      In the first argument of `(++)', namely `init x x'
      In the expression: init x x ++ newNmr x ++ y
    * Relevant bindings include
        y :: [a] (bound at C:\users\niel\desktop\test2.hs:41:21)
        x :: [a] (bound at C:\users\niel\desktop\test2.hs:40:21)
        z :: ([a], [a]) (bound at C:\users\niel\desktop\test2.hs:39:21)
        newNmr :: [a] (bound at C:\users\niel\desktop\test2.hs:34:30)
        lst :: [a] (bound at C:\users\niel\desktop\test2.hs:34:26)
        toReplace :: a (bound at C:\users\niel\desktop\test2.hs:34:16)
        (Some bindings suppressed; use -fmax-relevant-binds=N or
-fno-max-relevant-binds)
2)
 * Couldn't match expected type `[a] -> [a]' with actual type `[a]'
    * The function `newNmr' is applied to one argument,
      but its type `[a]' has none
      In the first argument of `(++)', namely `newNmr x'
      In the second argument of `(++)', namely `newNmr x ++ y'
    * Relevant bindings include
        y :: [a] (bound at C:\users\niel\desktop\test2.hs:41:21)
        x :: [a] (bound at C:\users\niel\desktop\test2.hs:40:21)
        z :: ([a], [a]) (bound at C:\users\niel\desktop\test2.hs:39:21)
        newNmr :: [a] (bound at C:\users\niel\desktop\test2.hs:34:30)
        lst :: [a] (bound at C:\users\niel\desktop\test2.hs:34:26)
        toReplace :: a (bound at C:\users\niel\desktop\test2.hs:34:16)
        (Some bindings suppressed; use -fmax-relevant-binds=N or
-fno-max-relevant-binds)
I've tried a lot, but I always got an error.
What am I doing wrong?
Thanks!
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Re: [Haskell] Function to replace given element in list

Richard Fung