Question

I am trying to convert nested lists into a custom type called Mydata using a list comprehension as follows:

main = do
    let a = [["12.345", "1", "4.222111"],
             ["31.2", "12", "9.1234"],
             ["43.111111", "3", "8.13"],
             ["156.112121", "19", "99.99999"]]
    let b = foo a
    print b

foo xss = [(xs,xs) | xs <- xss, xs <- xss]
    where
         xs = Mydata (read xs!!0 :: Float) (read xs!!1 :: Int) (read xs!!2 :: Float)

data Mydata = Mydata {valA :: Float, valB :: Int, valC :: Float}

When I run my program I get the following error:

1.hs:11:28:
    Couldn't match expected type `String' with actual type `Mydata'
    In the first argument of `read', namely `xs'
    In the first argument of `(!!)', namely `read xs'
    In the first argument of `Mydata', namely `(read xs !! 0 :: Float)'

Can anyone help me figure out what the problem is? Thanks.

Answer 1

xs definition in list comprehension is (perhaps unintentionally) recursive, and makes no sense. Possible implementation follows:

data Mydata = Mydata {valA :: Float, valB :: Int, valC :: Float} deriving Show

-- rely on type inference instead of specifying explicit type for each `read'
dataFromList [a, b, c] = Mydata (read a) (read b) (read c)
dataFromList _ = error "dataFromList: not enough arguments in list"

main = do
    let a = [["12.345", "1", "4.222111"],
             ["31.2", "12", "9.1234"],
             ["43.111111", "3", "8.13"],
             ["156.112121", "19", "99.99999"]]
    let b = map dataFromList a
    -- alternatively
    -- let b = [dataFromList triple | triple <- a]
    print b