Week 2: Git and First Functions

Transcription

1 Week 2: Git and First Functions Steven X. Han In this lab you will complete Lab 1 by submitting your work into git so that your tutor can access it. To do so successfully, read all explanations below very carefully. Afterwards you will learn some essential Haskell data types and constructs (guard expressions and case expressions) and write your own Haskell programs to practice them. Once you are done, remember to push the final version of your code via git before your next lab, so that your tutor can mark it. Remember to make an independent commit of your work for every exercise! It is normal that you don t finish the content during the lab, because you are expected to finish the rest of them as homework. Pre-lab checklist You need to know how to write simple functions in Haskell. You have to have finished Lab 1. Objectives In this lab you will... learn how to use git attempt writing functions with guard and case 1

2 Git So.. what is git? Git is a type of version control system (VCS), and as you can probably deduce from VCS, it keeps track of your progress as you work on a file. Think of it as an advanced Dropbox! If you are familiar with Time Machine backup on Mac computers, Time Machine is a type of version control system as well. If you want to read more about VCS and Git, Wikipedia has a very comprehensive article on it: Why don t we just use Dropbox then? Git provides very advanced version control, as such, it is very easy to revert back to any commit point in the past (provided that you have committed properly - we will go through this soon). It also provides an excellent interface from command line/terminal so that operations can be batch processed and you have full control over what s committed, what s pushed, and what s deleted. Usage In this section we will go through how to use git. Basic Concepts Repositories This is where your root folder in the cloud is called. For example, you can create a repository called comp1100-assignment-1, in which all the files related to your assignment 1 are. Similarly for subsequent assignments, you can create repositories for them as well. Adding files When you create a file (let s say.. mywork.hs) in a folder on your computer, it isn t necessarily in your git repository yet, because you haven t told git to manage that file. In order to tell git Hey, I want you to manage this file!, you just type in git add mywork.hs and git will now start managing this file. If you want to add everything in the directory, simply type git add. (notice the dot!), and this tells git to watch for everything in this folder. Commit your changes When you change something, and you want to save your current progress, you will need to let git know. It s almost like a save game function, but instead of going to a save point, you type in git commit -m "Some commit message." 2

3 The -m stands for with commit message, and the message inside the double quotation marks are your commit message (keep the double quotes!). Write something meaningful, such as Implemented areaoftriangle, testing needed.. instead of Commit 1.. The commit message should remind you of your progress, so instead of using your brain to store this information, you can store it on the computer. People with whom you have shared your files will also be able to see these messages and figure out where you are at. Important: Only commit code that compiles. Never commit code that does not. Push After you have added your files, committed your changes (which you can do without an internet connection by the way, because everything is processed locally), you will need to upload (push) all the progress you have made on your computer to the server, so your tutor, and you, when you have moved to another computer, will be able to see the files you have been working on, along with all the commits and commit messages. To do this, simply just type in git push Pull After you have pushed on your computer, and you go to a lab (or vice versa), you can download (pull) the stuff from the cloud down to your local computer, by typing git pull origin master Think of it as the exact opposite of pushing. The origin bit means you want to pull the stuff from origin repository, and master bit means you want to get the master branch (don t worry if you don t understand this right now, it will become clear to you as you use it more often). Merge If you have done everything properly, i.e., push every time you finish working on a computer, and pull every time before you start working on a computer, you should never need to merge anything since there shouldn t be any conflicts. In the unfortunate event that you do have to merge, I would recommend you to go to the following web page to get more information on merging since it s a bit too complicated to cover here: Fork Forking a repository creates an identical copy to the original repository, but if you change things on the fork, the original repository isn t affected. In this course you will need to fork the lab materials in order to obtain them, and the assignment documents, in order to do your assignments. Forking is done through the graphical user interface on GitLab. 3

4 ANU Environment For this course, we are using GitLab as our cloud. It is a nice interface to the git server, and does not much more than that. The eagle eyed ones amongst you would have noticed, we ve been talking about committing, pushing, and pulling, but where to/from? Git can t figure out magically which repository to push to, or pull from. It also doesn t know what you are committing to. In order to make this all work, we will need to do some foundation work. git init This command will initialise the current directory and set up necessary preliminary stuff for it to function as a git managed folder. git remote -v The repository in the cloud is called a remote, and this command lists all the remotes which are associated with the git repository in the current folder. (If it s blank, it means this current folder isn t linked up to any remote repositories) git remote add remotename remoteurl As you can probably guess, this adds a remote to the git repository in the current folder. We call the remote which we are about to add, remotename, although for the main remote, you should call that origin. For remoteurl, there are two formats, for accessing the remote via different protocols. For simplicity, we will be using the HTTPS protocol for now, but if you understand how to use ssh protocol, you might find that beneficial in some circumstances. So to add your main repository, you should type in something along the lines of git remote add origin you should of course replace uxxxxxxx with your GitLab username, which should be your uni ID. git remote add upstream upstreamurl After you have forked a directory, what happens when the original repository gets updated? Sometimes this isn t an issue, but most often, you will want that change to be in your fork. For instance, later in this lab you will pull the week 2 lab materials. In week 3, the lecturer will update the material in the original repository, and for obvious reasons you want to get the week 3 materials in your fork. What do you do? You add an upstream, from which the changes can flow down. In this case, the upstream of your repository is the course lab repository. Whenever something gets updated in the upstream, you can type in git pull upstream master in order to receive the updates. 4

5 Your turn! Okay that was a lot of reading! It s time for some magic Terminal actions. Open up IntelliJ, and in the Terminal window, it s your task to do the following, do feel free to refer to the sections above if you forgot which functions to use: 1. Make sure you are in ~/IdeaProjects/COMP1100 as we created in Lab Initialise the directory as a git repository 3. Open up the course s shared directory comp1100/labs.git, and click on Fork 4. Select your user, this will start the forking progress. Select User Afterwards, there will be a banner on the top, saying The project was successfully forked. Fork 5. Just below the repository name, there s a dropdown, defaulted to SSH. Select it, and change it to HTTPS. Copy the updated URL on the right of this button. This is the URL of your COMP1100 repository. 6. Back in your IntelliJ Terminal, add a remote, called origin, with the URL you just copied above. 7. Add the course s shared directory as the upstream repository. 8. Pull the files from the cloud down to your computer using git. Make sure that folder Lab02 appears locally on your machine after you pull. 9. Create folder Lab01 in your local git repository and add your Area.hs file which you made last week there. Commit this change with an appropriate message. Push the changes back to the cloud. Read and understand the terminal message that appears after you push. Act accordingly, if required. 10. Go back to your GitLab repo (in the cloud) to see if Lab01/Area.hs is now there. 11. Share your GitLab repository with your tutor as follows: 1. Click on Settings on the top right of the page Settings 2. Select Members from the sub-menu Members 3. In the Search for members to update or invite, type in your tutor s name, and click on the result. 5

6 4. Set the permission to REPORTER Reporter Access 5. Click Add to project Before proceeding, make sure that you successfully pushed the code you have implemented in Lab1 into the GitLab repo. Go to (substitute XXXXXXX with your ID) via your browser and observe the changes in the repository. Make sure that you shared your GitLab repo with your tutor. Make sure that you understand all git commands described above and what they do. You will use these commands for submitting your solutions to all labs and assignments. 6

7 Conditional functions The functions you ve been working with so far are just simple mathematical calculations involving their inputs. But you can also write conditional functions in Haskell. Such functions contain syntax that allows to select among answers depending on the truth of certain conditions. There are several options for these syntactic structures in Haskell two important ones are case expressions and guarded expressions, also known as guards. We will start with guards, and we will learn case expressions later in the lab. Booleans Boolean is a data type that can be one of two values - True or False. Guards rely extensively on Booleans. A Boolean result is produced when the program evaluates a statement given by you. They can also be chained together, and produce more interesting Booleans. Try the following in GHCi (one line at a time): == 2 2 * 2 == == 2 2 * 2 == == 2 && 2 * 2 == == 2 && False True && True True && False False False True (False && True) 7

8 Exercise 1 - Guards Let s start by writing a function which replicates the following idea: I m running a supermarket, and planning on doing a promotion on premium ramen products. I have decided (against my consultants unanimous disagreement) to charge people less per packet the more packets they buy. The price guide can be found below. No of packets 1-5 $ $ $ $ $ $1.3 Price per packet Your job is to design a function, called totalprice, which takes the number of packets as the argument and outputs the total amount that should be charged. If you have previous programming experience with imperative languages (python, c, java, etc.), you will automatically think about if statements. However, despite its existence in Haskell, one should rarely use if as there are more elegant solutions out there which require less effort. Your if statements may be structured as the pseudo-code (code which conveys meaning only but doesn t strictly comply with any programming language syntaxes) below: totalprice (Integer x): if 1 <= x <= 5 then return (2 * x) else if 6 <= x <= 10 then return (1.8 * x) Guards can be thought of as a structure of if/then/else. Observe their syntax in the following example: grades :: Integer -> String grades x x >= 80 && x <= 100 = "High Distinction" x >= 70 && x < 80 = "Distinction" x >= 60 && x < 70 = "Credit" x >= 50 && x < 60 = "Pass" x >= 0 && x < 50 = "Fail" otherwise = error "Something's wrong." 8

9 someboolean = someresult is the basic syntax of guards. If unsure about Booleans, go back to the Boolean section and review. You can find function grades in Grades.hs that you pulled from the cloud repo. Load this function into GHCi (for this type ghci -W Grades.hs), and play with it. Try various values of the integer argument. With the assistance of the example code above, can you work out how to implement function totalprice you are asked to do? Implement this function in a separate file called Price.hs. Submission required: Exercise 1 9

10 Exercise 2 - Elegant Guards Haskell has this wonderful (you will love it once you truly understand it) concept of lazy evaluation. Let us explain this to you with an example (Lazy.hs): toolazy :: Integer -> String toolazy x x < 10 = "Input is less than 10" x < 5 = "Input is less than 5" Load Lazy.hs to GHCi. What happens when the input you ve given it is 8? What happens when the input is 3? Is this what you have expected? If so, great! If not, this is lazy evaluation in action! Haskell will find the first guard that satisfies the condition, and will ignore everything below. We can leverage on this and make our codes a bit simpler and more elegant. 1. Create the improvedgrades1 function in Grades.hs without using &&. In other words, write improvedgrades1 function with only one conjunct in the logical condition within each guard! 2. Write more elegant improvedtotalprice function in Price.hs to satisfy the requirements above. Submission required: Exercise 2 10

11 Exercise 3 - Guards are exhausted... or are they? For your totalprice and improvedtotalprice functions, what if a customer comes in and buys... no ramen noodles? Or some desperate student comes in, trying to be difficult, and buys 1001 packets? Do your functions handle these? Handling the 0 case is easy - if you don t buy any ramen noodles, you don t get charged! For the 1001 case, I... have to confess, I tricked you in the instructions. The instructions given to you simply did t specify what happened if they bought more than 1000 packets. Therefore it s not your fault! However, you should handle this situation somehow. You can see in the grades function, there is an otherwise clause. otherwise is synonymous to True when they are used in guards. So think back on the lazy evaluation property that we talked about, it will get executed if none of the Boolean expressions above it evaluate to True. You can implement an otherwise clause to your improvedtotalprice function, and throw an error when they try to buy more than 1000 packets. To throw an error, refer to the grades example. Now edit your improvedtotalprice function to handle the situations where a customer buys none, or buys more than 1000 packets. What happens if your improvedtotalprice function is called with a negative argument? Submission required: Exercise 3 11

12 Exercise 4 - Case You are not required to understand the code that s given to you in this exercise. Open up the file named Week.hs. In it, you will find our custom data type Day, which can be one of seven things, each representing a day of the week. The function showtheweek will call getname function (which you will implement) with every day of the week one by one, and prints them in the Terminal nicely. Your job is to write a function getname, which takes in our custom data type Day as an argument and outputs the String of that day s full name, e.g., getname Mon should return Monday. Take 5 minutes and try to implement this yourself. A pause in this document will be added for dramatic effect. << Pause >> You may have failed miserably by yourself. Don t worry, we are here to help! If you tried to use Guards to implement this, you might come across this error message: error: No instance for (Eq Day) arising from a use of == In the expression: day == Mon In a stmt of a pattern guard for an equation for getname : day == Mon In an equation for getname : getname day day == Mon = "Monday" This is because as Day is a custom data type, it is not automatically Equitable. It s like saying Chair A is equal to Chair B. How do you compare the chair? Do you compare the shape? The colour? The smell? The butt print?? However, when we are looking at a chair, we know, that it s a chair. How? Patterns! Chairs normally have a back, a place to sit, four legs, and sometimes arms. As long as we are certain of the rough definition of a chair, then we can generally conclude that it s a chair when we are looking at a chair-like object. Haskell can do similar things, but in a much better way. We can use case statements to do Pattern matching. For example, the getname function can be started with: getname :: Day -> String getname day = case day of Mon -> "Monday" The structure of case statements are: 12

13 case something of Pattern1 -> dothing1 Pattern2 -> dothing2 Pattern3 -> dothing3 Note that something can be an argument that was passed into the function (in our example, the day argument), or it can be a function call which returns something that you want to pattern match against (we will go into this in more detail later in this course). Can you now finish the getname function now? Submission required: Exercise 4 Just in case... The eagled eyes amongst you might have questioned - if guards can be nonexhaustive, can cases be too? The answer is - of course! What if the something matches against none of the patterns provided? Instead of otherwise in guards, we will use an underscore _ to represent Any pattern. Lazy evaluation rule also applies here. Question: Do you need to add a _ case for getname? Why or why not? 13

14 Exercise 5 - Maybe... Remember your last implementation of improvedgrades1. Since there is no grade for integer values below 0 and above 100, we displayed an error message in GHCi for these cases. This approach would not be ok if you programmed a flight controller just imagine the controller stopping flying the plane to display a message on the screen with an explanation why it cannot continue flying! There are several other methods to handle exceptional values or situations. One of them is to use special types that can hold some exceptional values on top of their normal value range. The simplest of these is the Maybe type, which can hold one special value Nothing (Note: Nothing is a value and not a type). This allows us to define a function that returns (or accepts) values of a specific type, but also has the option to say that it cannot return anything by returning value Nothing instead. So, the Maybe type is pre-defined in Haskell and it looks like this: data Maybe a = Nothing Just a where a stands for any type. First of all, let s define a new custom data type just like we did for Day: data Grade = HighDistinction Distinction Credit Pass Fail Your job is to write new function called improvedgrades2 function such that instead of a String, it returns a Grade, the data type we just defined. Your next task is to use the Maybe type to stop this function throwing an error when the marks put in are out of bound. Write function improvedgrades2 with the type signature improvedgrades2 :: Integer -> Maybe Grades Note that when the mark is within bounds, instead of a member X of the datatype Grades, we now return Just X. When the mark is out of bounds, we now return Nothing. Test your implementation of improvedgrades2 and see what GHCi displays as return values. Other numerical types use a similar concept. For instance, try to divide 1 by 0 in GHCi and see whether the output is what you expect: Prelude> 1 / 0 Submission required: Exercise 5 14

15 Exercise 6 - Maybe may be useful... maybe. Here s another problem. At some point, you will want to figure out what GPA corresponds to which Grade. To simplify things, here s a table modified from Grade Description Grade Point Value HD High Distinction 7 D Distinction 6 CR Credit 5 P Pass 4 N Fail 0 It would be handy if you could enter your mark, and it automatically calculated the GPA associated with that mark. e.g. if marktogpa 80 returned 7 as the result. Let s break this down to two steps. We have already written the part where you convert your mark to a grade. The next step is to write another function to convert grades to GPA. Therefore the new function should have the type signature of Maybe Grade -> Integer. You have already learnt that Maybe type is defined as follows: data Maybe a = Just a Nothing. How do you actually utilise this in a productive way? Let s start with an example: maybetoint :: Maybe Integer -> Integer maybetoint m = case m of Just i -> i Nothing -> 0 The example above takes in a Maybe Integer which is either Just Integer or Nothing. If it follows the pattern of Just <some integer value i>, return i, if it s Nothing, return 0. Have a closer look at the third line of this example - what is i? What s the data type of i? Your next task is to write a function maybegradetogpa :: Maybe Grade -> Integer in Grades.hs. (You will have to nest case statements!) Note, if the Maybe Grade is Nothing, return 0. Submission required: Exercise 6 15

16 Exercise 7 - Finally!... almost. Let s link all of these stuff up together! Let s list out the functions you ve already written and are going to use: * improvedgrade2 :: Integer -> Maybe Grade - This takes a raw mark and outputs a grade (HD, D, CR, P, N) * maybegradetogpa :: Maybe Grade -> Integer - This takes a Maybe Grade and outputs a GPA associated with that grade. What s the next step? Your final task is write a function: marktogpa :: Integer -> Integer, which takes in a raw mark in Integer (0 to 100) and outputs a GPA (0 to 7) associated with that mark. Submission required: Exercise 7 16

17 Exercise 8 - One last thing... Doctest. Just because your code compiles, doesn t mean it s doing what it s supposed to do. Consider the following: addone :: Integer -> Integer addone x = x + 50 Does this compile? Of course it does! Does this do what it s supposed to? Deducing from the name, it should take a number, and add one to it. However, what it actually does is taking a number, and add 50 to it. For simple functions like these, it s relatively easy to figure out that it s wrong before even compiling it. For more complicated functions, however, it is actually easier to compile it and simply run the code with a pre-determined input and expected output, and see if it does what you wanted it to. For example, if I compiled the above code and typed into GHCi: addone 5, I would expect 6 to be the answer, but it would give me 55. There is a very powerful tool in Haskell, which takes away the manual labour of testing everything yourself. You simply tell it what to do, what to expect, and it will check the functions for you. This is Doctest, which we will extensively use throughout the semester. What does a Doctest statement look like? main :: IO() main = undefined -- Adding one >>> addone >>> addone addone :: Integer -> Integer addone x = x + 50 If you copied and pasted the above code into a file called Adding.hs in IntelliJ, then in the Terminal (Notice, not in GHCi! If you are in GHCi, type :q to exit into Linux environment), type in doctest Adding.hs, it will give you the following output: > doctest Adding.hs ### Failure in Adding.hs:9: expression `addone 5' expected: 6 but got: 55 Examples: 2 Tried: 1 Errors: 0 Failures: 1 17

18 Notice that there was a main function, this is simply to make Doctest happy, not functionally anything. If you didn t have the main function, Doctest will complain and refuse to run! Your job is to edit your Week.hs, and Price.hs, such that they both utilise Doctest to ensure the functional correctness of your functions. Submission required: Exercise 8 18

19 Written by Steven X. Han. Some parts are modified from materials produced by Tony Hosking and Uwe Zimmer. 19