functional programming in Haskell

[manish411]

I have started functional programming in Haskell and I need a mentor
to clear some basic doubts. Plz someone help me!!

[simeon87]

Could you post your questions here? That's usually easier than asking for a mentor. I know that several people here know Haskell (including me, to some extent).

[manish411]

I am writing a basic functions which calculates ave of marks given in a list
<code>
percent a =
let sum = addition a
per = fromIntegral sum/fromIntegral $ length a
in per
</code>

but I am getting an error

No instance for (Fractional (Int -> b))
arising from a use of `percent' at <interactive>:1:0-17
Possible fix:
add an instance declaration for (Fractional (Int -> b))
In the expression: percent [23, 44, 55]
In the definition of `it': it = percent [23, 44, 55]

[simeon87]

Not sure what's going on there but it looks like you're giving a function where a number is expected. Can you try:


fromIntegral sum / fromIntegral (length a)

I must say it has been a while since I did Haskell.

[manish411]

thanks it worked,but what was the problem??

[simeon87]

The problem is:


fromIntegral sum/fromIntegral $ length a

It's wrong to have a $ in that location, it messes up the types. You can use $ to correctly do function composition, such as:


funcOne.funcTwo.funcThree $ arg

This will cause Haskell to do the function composition first (combine funcOne, funcTwo and funcThree into one function) and then apply that combined function to arg. That's what I know about using $. In your case you have


fromIntegral sum / fromIntegral

on the left side which is weird because you can't divide by a function (which is what Haskell will try to do due to the $).

[manish411]

Now I want to modify the code and want to include IO actions


percent a =
let sum = addition a
per = fromIntegral sum /fromIntegral (length a)
in per

main = do
putStrLn " Enter the marks in a list"
marks <- getLine
print $ percent fromIntegral $ marks



However the getLine function is the functions which binds the input which is of type String
There are different functions for taking input of different types .
What if we want an input of a type which is of Int, a list , an algebraic data type?

[simeon87]

What you'll get from getLine is a String. That means you'll need to convert it to a different data type yourself with a parser.

[schauerlich]

Please use [code] tags when you post code (those are square brackets, not angle brackets).

[manish411]

This wont work either:




addition x:xs
| x ==[] = 0
| otherwise = x + addition xs

--percent :: [Int] -> Float

percent a =
let sum = addition a
per = fromIntegral sum /fromIntegral (length a)
in per

main = do
putStrLn " Enter the marks in a list"
marks <- getLine
let mark = (read marks :: [Int])
print $ percent mark

error
Prelude> :load functions.hs
[1 of 1] Compiling Main ( functions.hs, interpreted )

functions.hs:1:0: Parse error in pattern
Failed, modules loaded: none.

[schauerlich]

addition x:xs
| x ==[] = 0
| otherwise = x + addition xs

--percent :: [Int] -> Float

percent a =
let sum = addition a
per = fromIntegral sum /fromIntegral (length a)
in per

main = do
putStrLn " Enter the marks in a list"
marks <- getLine
let mark = (read marks :: [Int])
print $ percent fromIntegral $ mark



You need parentheses around the (x:xs) when it's in a pattern match, that's why it fails. In (x:xs) :: [a], x must be of type a, and xs must be of type [a]. You try to compare x, which is some integer, to the empty list. You want something like this:


addition :: (Num a) => [a] -> a
addition [] = 0
addition (x:xs) = x + (addition xs)