Haskell - Reading a file and passing data as arguments to function

Question

I'm trying to get some data from a file, then parse it and pass it to another function as an argument.

data LogLine = LogLine {
  name      :: String
, args1     :: String
, args2     :: String
, constant  :: String 
} deriving (Ord, Show, Eq)

main = do 
file <- readFile "foo"
let result = (parse final "Input" file) --Parses the file into the LogLine datatype
let firstargs = getFirstArgs result --Get the first argument out of the datatype
let secondargs = getSecondArgs result --Get the second argument out of the datatype
let constant = getConstant result --Get the constant out of the datatype
createGraph firstargs secondargs constant --THIS IS THE PROBLEM

The problem is that whenever I try to read-in a file it becomes an (IO String) and I always have to carry the IO whatever I do. the createGraph function is declared as

createGraph :: String -> String -> String -> Argument

but whenever I try to execute the last statement it complains:

Couldn't match expected type `IO a0' with actual type `Argument'
In the return type of a call of `createGraph'

I'm not allowed to change the return type of the createGraph function, because it's a part of a large framework that I need to feed the arguments to. What are the ways of dealing with this?

Something is not matching between your code and your description. You call createGraph in your code and reference createArgument in your description. Assuming they're the same function the problem is that the return type of main must be IO () but the return type of createGraph is Argument. — asm
– asm, Commented Oct 9, 2012 at 0:19
Sorry, that's a typo. They're both the same function, I wanted to rename them for the sake of explicitness. — Nasco Bozhkov
– Nasco Bozhkov, Commented Oct 9, 2012 at 0:24
Is you main function in your code snippet the main for your program? If so then the final expression in the do block must have type IO () — asm
– asm, Commented Oct 9, 2012 at 0:32

user1105045 · Accepted Answer · 2012-10-09 00:35:37Z

2

Why would you want to do that?

The only way to get your value into the IO monad is by using return. You can either wrap the call to createGraph into another function like

returnGraph a b c = return $ createGraph a b c

or just use another let binding and use your value when you need it. I can't figure out what you want to do there.Please give us more details as in what do you want to do with the returned value.

-- From what i understand from your comment you just need to return the argument so the only thing you have to do is return $ createGraph firstargs secondargs constant and rename the function from main to something else because main must have type IO ().

edited Oct 9, 2012 at 0:35

answered Oct 9, 2012 at 0:22

user1105045

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2 Comments

Nasco Bozhkov Over a year ago

I've a framework which needs to get arguments in order to work. What I'm trying to do is parse the arguments from a file into Strings and then pass them to the framework in order for it to compute the result. What I want to do is the following - have the framework call the main function so that it parses the file and returns the arguments needed. So the only way to do that (which I see) is to have that do block parse the file and call the framework functions in order to start it.

Nasco Bozhkov Over a year ago

Okay, I warped it, like you suggested and it works now. Thank you!

Code-Apprentice · Accepted Answer · 2012-10-09 00:35:38Z

1

The problem is that whenever I try to read-in a file it becomes an (IO String) and I always have to carry the IO whatever I do.

I don't think this is the real problem. The problem is that main has a return type of IO() which is the result of the last line executed. In this case, that means the createGraph call which results in an Argument. This is why you get the type error, which has nothing to do with the IO String that is read from the file.

One solution is to simply return the result of createGraph at the end of main:

return $ createGraph firstargs secondargs constant

answered Oct 9, 2012 at 0:35

Code-Apprentice

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