Question

First off I am using ghci under ubuntu 11.10 to run the haskell code. 2nd this is my first attempts at haskell. Third, how might I load a file into ghci and where does it need to be located and what should its extension be? I know “:l “file.haskelxtnsn”” is how to load a file, but that’s my best guess so far.

Seeing as I can do the above, how does this code look for merging two list of possibly infinite size in ascending order. (I can’t put this in the prelude> prompt because of indentation???) Given [1, 2, 3] and [4, 5, 6] I should get [1, 2, 3, 4, 5, 6], and I think the usage would be “take 10 (merge listx listy)”

let merge x y = (min (head x) (head y)) : 
     case (min (head x) (head y)) of 
         head x -> merge (drop 1 x) y 
         head y -> merge x (drop 1 y)

psuedo:

• output the min of the heads of the lists
• if the first lists head was output call merge with the rest of the first list and the second
• else call merge with the first list and the rest of the second list

Answer 1

• Usually the extension used is “.hs”.
• You can use :cd in ghci to change directory, you can also supply a path to the :load (:l for short) command.

• Your logic is correct, although maybe I’d write it a bit differently (hopefully you know about and where clause and defining a function as a series of equations):

  merge [] ys = ys
  merge xs [] = xs
  merge xs ys = min x y : if x < y then merge (tail xs) ys
                                   else merge xs (tail ys)
          where x = head xs
                y = head ys
  

• In ghci you need a let in front of definitions, which is different from the let ... in ... expression. This is rather confusing so I suggest you just put your code in a file and load it in ghci.
• Function application has higher precedence then the : operator, so some of you parenthesis is not needed. We usually try to minimize the number of parenthesis to make the code more concise, but don’t be over zealous about it.
• I don’t really see the point of using a case expression here (other than causing an error). Try reading on pattern matching for more detail, data constructors vs function applications, why you can’t use head x inside a pattern but you can do x:xs (Although I didn’t here). Calling head and min multiple times looks redundant, andy ou can also substitute drop 1 with tail.