Bash-Shell.Net

It's been awhile, but day 2 of Haskell is complete. I've got a lot less spare time for doing these now that classes have changed up. I'm at work full time and then in class from 6pm to 10pm three days a week plus doing homework, etc. The house also got struck by lightning (everyone is ok and even the damage isn't too bad, all things considered) earlier this week which added more delays.

The first exercise was fairly simple, write a function that takes a list and returns a sorted list. I kind of feel like I cheated on this one, though. I knew the logic that I wanted to use but had to go to google for the syntax. Unfortunately that pretty much gave me the entire answer since it's so short. My base logic was correct though, so that's good.

This function takes the head of a list and then passes it into the filter function looking for all members of the list that are less than the head and then continues to do that recursively for each of those so they are in order and prepends them to the head(s). Then the same is done appending to the head(s) for list members greater than the current head.

Next up was to do the same thing but allow the user to pass their own function into the sort function as a parameter.

The third exercise was to write a function to convert a string in the form of $2,345,678.99 with any number of leading 0s to a number. In most languages that I'm used to working with this probably would have been done with a regular expression, but it's handled a bit differently in Haskell.

Number four is a bit odd.I suspect the exercise is poorly worded. The exercise is to write a function that takes parameter x and returns a lazy list of every 3rd number and another that takes parameter y and returns a lazy list of every 5th number. Then it says to use composition to combine the two functions to create a lazy list of every 8th number starting with x+y.The problem here is that composition, the. operator, used like f . g, passes the return value of g into function f. Since both of these functions return a list but only take an integer I don't see how you could use composition to combine them. I asked another more experienced Haskell developer and he felt the same as I do. The solution I used with the zipWith function is also how everyone else seems to have completed this exercise.

Note that inc3 and inc5 could also have been done with range and pattern rather than recursion. I went with recursion because that's what the author of the book used in his lazy list examples and I wanted to be sure I understood the concepts he was trying to get across.

Excercise 5 said to write a partially applied function that takes a parameter and returns half and another that appends \n (an actual newline, not a string literal \n). The partially applied function concept is simple enough, you can create a function that is another function plus just some of the parameters it requires. Then whatever you pass as the parameter to the "new" function gets used as the final parameter of the original function. Haskell also lets you use prefix operators if you want to which makes writing partially applied functions really simple.

There are also a few more exercises that the author suggests if you want something more challenging.

The first of the more challenging exercises is to write a function which takes two integers and returns the greatest common denominator. The built in gcd function was written as a recursive function using some extra Haskell syntax that I am unfamiliar with (or was, after seeing it I learned about it and used it in the next exercise) and is nearly instant even with large numbers while mine has a small delay with larger numbers. The numbers in the example have maybe 1/2 second delay due to generating the lists and then comparing them with intersect and bigger numbers continue to get slower.

Next is generating a lazy list of prime numbers. I never would have come up with the solution for this on my own. I looked up algorithms for finding prime numbers but still didn't have the knowledge of Haskell to implement them. It seems like these might be using a library that I don't have installed, Data.List.Ordered, because I can't import that module and do not have the unionAll function. This wikipedia page has Haskell implementations of the Sieve of Eratosthenes and Euler's Sieve. I was going to try to create a lazy version of Euler's Sieve but the lack of the minus function has stopped that.

The last bit was broken up into a few steps, but I'm posting them all as one program. First was to create a function that takes a string, breaks it up on word boundaries to add a new line, then returns it. After that, add line numbers. Then after adding line numbers, add functions to left, right, and center align the text using spaces for padding.

I had a couple of ways to go about adding the padding. My first thought was to just use printf, but sorting out how much padding to add actually turned into more hassle than I initially expected. I ended up just writing functions to pad to the right or left which took how many spaces to add as a parameter. I chose to add more spaces to the right than left when an uneven number of spaces was needed. For left and right alignment I just combine the output of get_left_padding and get_right_padding which each return just half the padding needed for the whole line. I also just align with the length of the longest word since I don't have set width such as for screen or a piece of paper.