CS 331 Spring 2009
Additional Lecture Notes for Wednesday, January 28, 2009

The following material was covered in class, in addition to the material from the text.

A Brief Introduction to Haskell

Getting Haskell

Later in the semester, we will be looking at the programming language Haskell (named after logician Haskell Curry). You will need to get access to some form of the Haskell language. Some options:

• Hugs. An interactive semi-interpreted (hybrid) system. RECOMMENDED.
• GHC. The Glasgow Haskell Compiler. This is the most popular Haskell compiler.
• Other Haskell compiler. There are several.

Please avoid “Helium”, due to some incompatibilities.

I have asked for Hugs to be installed in the Chapman 103 lab.

Using Hugs

Hugs is a semi-integrated interactive system. It does not have its own editor; you may need to configure it. At the Linux prompt, if you run Hugs by typing

hugs -EEDITOR_NAME

hugs -Egvim

At the Hugs prompt, if you type something starting with a colon (“:”), it will be taken as a Hugs command. Anything else is a Haskell expression. Type “:?” for a list of commands; these can be abbreviated.

To edit a file, type

:edit FILENAME

:e test1.hs

Once you have written some code in your editor and saved it (if possible, keep the editor running), you load it into Hugs with “:load”:

:l test1.hs

If you change the code in the file, you can redo the last load command using “:reload”.

:r

Sample Haskell Code

Here is some Haskell code similar to that written in class. It defines a function fibo, which takes an integer parameter n and returns the nth Fibonacci number.

-- fibo: compute a Fibonacci number
fibo n = fst (fibox n)
    where fibox 0 = (0, 1)
          fibox n = (b, a+b)
              where (a, b) = fibox (n-1)

This only defines a function, and so loading it into Hugs will not appear to do anything.

However, after you load it, you can use “fibo” in expressions. For example, if you type

fibo 10

fibo 1000
fibo 1000000
[fibo n | n <- [1..10]]

• Indentation is significant.
• A function’s parameter simply follows the function name. No parentheses are necessary.
• However, you may need parentheses to deal with precedence correctly. Function application generally has higher precedence than arithmetic operators.
• Function definition can use pattern matching. Above, there are two definitions given for fibox. A parameter of 0 matches the first; anything else will move on to the second.
• You usually do not need to specify types. Do not get the idea from this that Haskell does not care about types. On the contrary, it is pickier about types than other any language I know. However, it is usually able to figure out what the type of something needs to be, in which case you do not need to specify the type.
• The “where” construction lets you define local functions and such.
• Use “--” to begin a one-line comment.
• Haskell’s integers have no arbitary limits. However, you can still run out of memory.

We will do more with Haskell later in the class. If you want to learn before then, there are a number of good tutorials on the Web.

CS 331 Spring 2009: Additional Lecture Notes for Wednesday, January 28, 2009 / Updated: 28 Jan 2009 / Glenn G. Chappell / ffggc@uaf.edu