How to check my haskell functions

Question

I just started with Haskell and first tried to write some tests. Basically, I want to define some function and call this function to check the behavior.

add :: Integer -> Integer -> Integer add ab = a+b -- Test my function add 2 3 

If I load this little script in Hugs98, I get the following error:

 Syntax error in declaration (unexpected `}', possibly due to bad layout) 

If I delete the last line, load the script and then type "add 2 3" in the hugs interpreter, it works just fine.

So the question is, how can I put my function calls in the same script as a function definition? I just want to download the script and be able to check if it does what I expect ... I don’t want to enter them manually manually.

Answer 1

Make a top-level definition:

 add :: Integer -> Integer -> Integer add ab = a + b test1 = add 2 3 

Then call test1 in the Hugs session.

Answer 2

Others said how to solve your immediate problem, but for testing you should use QuickCheck or some other automated testing library .

 import Test.QuickCheck prop_5 = add 2 3 == 5 prop_leftIdentity n = add 0 n == n 

Then run quickCheck prop_5 and quickCheck prop_leftIdentity in a Hugs session. QuickCheck can do a lot more than that, but it will get you started.

(Here's the QuickCheck Tutorial , but it's out of date. Does anyone know the one that covers QuickCheck 2?)

Answer 3

The friendliest way for beginners is probably the doctest module. Download it using the “cabal install tutorial,” then put your code in the “Add.hs” file and run “doctest Add.hs" from the command line.

Your code should look like this: formatting is important:

 module Add where -- | add adds two numbers -- -- >>> add 2 3 -- 5 -- >>> add 5 0 -- 5 -- >>> add 0 0 -- 0 add :: Integer -> Integer -> Integer add ab = a+b 

HTH Chris

Answer 4

How can I put my function calls in the same script as a function definition? I just want to download the script and be able to check if it does what I expect ... I don’t want to enter them manually manually.

In short, you cannot. Wrap it in a function and call it instead. Your file serves as a valid Haskell module, and the expression "flying" is not a valid way to write it.

You seem to come from a scripting language, but don't try to consider Haskell as one of them.

Answer 5

If you have ghc installed, the runhaskell command will interpret and run the main function in your file.

 add xy = x + y main = print $ add 2 3 

Then on the command line

 > runhaskell Add.hs 5 

Not sure, but the hugs probably have a similar function for the runhaskell . Or, if you upload the file to the hugs interpreter, you can simply run it by calling main .