[Haskell] Function to replace given element in list

[Richard Fung]

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 at 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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