Higher order functions in haskell error - haskell

Question

selectMenu :: Int->IO()
selectMenu num
|(num==1)=convertFromDecimal
|(num==2)=--menu2
|(num==3)=putStrLn("3")
|(num==4)=putStrLn("4")
|(num==5)=putStrLn("5")
convertFromDecimal:: IO()
convertFromDecimal= do
putStrLn("\n\tConvert From Decimal To Binary & Octals \n")
putStrLn("----------------------------------------------------------\n")
putStrLn("Enter 5 decimal numbers [,,] : ")
input<-getLine
let n=(read input)::[Int] -- is this right?
--putStrLn (show n)
let result = convertionTO decToOct n
putStrLn(show result)`
decToOct :: Int -> [Int]
decToOct x = reverse(decToOct' x)
where
decToOct' 0 = []
decToOct' y = let (a,b) = quotRem y 8 in [b] ++ decToOct' a
convertionTO :: (Int -> [Int] -> [Int]) -> [Int] -> [Int]
convertionTO _ [] = []
convertionTO f (x:xs) = f x : convertionTO f xs
I correct those mistakes. I did update the question after correcting those errors. But this time it gives this error
How can i fix this error?
Assignment.hs:49:51:
Couldn't match expected type `[Int] -> [Int]'
with actual type `[Int]'
Expected type: Int -> [Int] -> [Int]
Actual type: Int -> [Int]
In the first argument of `convertionTO', namely `decToOct'
In the expression: convertionTO decToOct n
Assignment.hs:66:25:
Couldn't match expected type `Int'
with actual type `[Int] -> [Int]'
In the return type of a call of `f'
In the first argument of `(:)', namely `f x'
In the expression: f x : convertionTO f xs

Answer 1

(I'm copying the errors here in case you edit the question again.)
The first error
Assignment.hs:49:51:
Couldn't match expected type `[Int] -> [Int]'
with actual type `[Int]'
Expected type: Int -> [Int] -> [Int]
Actual type: Int -> [Int]
In the first argument of `convertionTO', namely `decToOct'
In the expression: convertionTO decToOct n
refers to this line of code
let result = convertionTO decToOct n
conversionTO expects its first argument to have the type Int -> [Int] -> [Int], but decToOct instead has the type Int -> [Int].
The second error
Assignment.hs:66:25:
Couldn't match expected type `Int'
with actual type `[Int] -> [Int]'
In the return type of a call of `f'
In the first argument of `(:)', namely `f x'
In the expression: f x : convertionTO f xs
refers to this line of code
convertionTO f (x:xs) = f x : convertionTO f xs
convertionTO produces an [Int], so the first argument to : must be an Int. But f x instead has type [Int] -> [Int].
How to fix these?
I'm going to assume that your line 49 is correct, and that the type signature for convertionTO is wrong. In that case it should be
convertionTO :: (Int -> [Int]) -> [Int] -> [Int]
This doesn't fix the second error, as f x now has type [Int].
The problem here is your use of :. : joins a single element to the start of a list. What you have is two lists that you want to join together. For this, use ++.
convertionTO f (x:xs) = f x ++ convertionTO f xs
Finally, note that your convertionTO is in the standard library as concatMap, which combines map with concat.

Answer 2

Those errors mean that you're referring to functions which don't exist. Either because you mistyped their names, or because you haven't defined them yet.
For example, instead of showresult, you probably meant show result, and I think you meant decToOct' instead of decToAny'. I can't see any references to convertToDecimal anywhere in the code you posted, so that error is probably somewhere else.