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Getting started with Hugs on LinuxCS190 Functional Programming Techniques

Dr Hans Georg Schaathun

University of Surrey

Autumn 2008Week 1

Dr Hans Georg Schaathun Getting started with Hugs on Linux Autumn 2008Week 1 1 / 43

Outline

1 Learning Objectives

2 File tree and command line

3 Software

4 The editor

5 Haskell


Learning Objectives

Outline



3 Software

4 The editor

5 Haskell


Learning Objectives

Learning ObjectivesFrom the module description

At the end of the module, the students willbe able to work with standard command-line interfaces.write programs employing the fundamental features of Haskell.be able to make and use reusable functions as a fundamentalbuilding block of software development.understand recursion and polymorphism and be able to use themin simple algorithm design.contrast functional and imperative languages and understand theadvantages and disadvantages of each.


Learning Objectives

The lab exercises

Weekly lab sessions – one exercise set per weekLab Sheet 1 for next week (preparatory exercises).

PortfoliosContaining Haskell functions from the weekly labsMust be passed

Portfolio 1 (Lab Sheet 2 for Weeks 2-4).Public test script (TBC)

Portfolio 2 (Lab Sheet 3 for Weeks 5-8).Secret test script (TBC)

Exact details will be announced later.Including assessment criteriaSee http://www.computing.surrey.ac.uk/teaching/current/cs190/


http://www.computing.surrey.ac.uk/teaching/current/cs190/

http://www.computing.surrey.ac.uk/teaching/current/cs190/

Learning Objectives

The exam

The mark is decided purely by the exam... though you will fail if you fail on a portfolio

2h written, unseen examYou need the practical understanding from the labs

The exercise material is central for the examIf you master the exercises, a catastrophe is unlikely

... but the exam will have something extraExtra exercises will help you


Learning Objectives

This session

This sessions will give you the basic background tostart using the faculty linux systemstart using the hugs Haskell interpreter

During this week you have totry the software in the linux labsmake sure that you can use the knowledge from the lecture


Learning Objectives

References

A basic book: Programming in Haskell by Graham HuttonA comprehensive book: Haskell: The Craft of FunctionalProgramming by Simon Thompson

You’ll need one of the bookshttp://www.haskell.org

You can download a copy of hugs

Reference on Unix/Linux (e.g. Unix in a Nutshell [O’Reilly])


http://www.haskell.org

File tree and command line

Outline



3 Software

4 The editor

5 Haskell



The command line (shell)

Open a terminal windowApplications→ System→ Terminal... emulates an old-fashioned text terminal (tty)

An interpreter (or shell) runs in the terminalreads and interprets commands

Different interpreters are available: bash, tcsh, ksh, zsh



The file tree

research/ teaching/Notes/ labs/

Mail/ bin/ Papers/ work/Mail/ cs190/ cs168/

css1hs/ css1ht/eps351/

bin/ lib/cscssst/ epsguest/

filer2/ filer6/ bin/ man/ local/

home/ tmp/ usr/

/



Navigating in the tree

The files are organised in a treeEach node is either

a directory, containing files and/or other directoriesa file, containing data for some piece of software

Current Working Directoryyour location in the tree

Useful commands:pwd : print current directorycd : change directoryls : list directory contents



Naming a node

A file/directory is identified by its pathName each directory traversed to reach the file

separated by slashes (/)A relative path starts from the current directory

e.g. cs190/test.hs.. identifies the parent directory

Absolute paths starts at the root (/)/home/filer6/epsguest/eps351/cs190/test.hs



Path: /home/filer6/epsguest/eps351/cs190/

research/ teaching/Notes/ labs/

Mail/ bin/ Papers/ work/Mail/ cs190/ cs168/

css1hs/ css1ht/eps351/

bin/ lib/cscssst/ epsguest/

filer2/ filer6/ bin/ man/ local/

home/ tmp/ usr/

/



Modifying the tree

mkdir : make new directoryrmdir : remove (empty) directoryrm : remove filecp : copy filemv : move file

For more information you can use the man pagesman cpman man

It takes some time to learn to read the man pages though



File information

% ls -ltotal 12drwxr-sr-x 2 eps351 epsguest 4096 Aug 6 13:25 labs/drwxr-sr-x 2 eps351 epsguest 4096 Aug 6 13:25 Notes/-rw------- 1 eps351 epsguest 0 Aug 6 13:26 test2.hs-rw-r--r-- 1 eps351 epsguest 8 Aug 6 13:25 test.hs

File type: d for directory, - for regular fileNine characters for mode (access permissions)Number of links to the fileFile owner (eps351) and group (epsguest)File size (bytes)Time of last modification



File mode (access permissions)

drwxr-sr-x 2 eps351 epsguest 4096 Aug 6 13:25Notes/Three user classes:

user/owner (char 2-4)group (char 5-7)others (char 8-10)

Three access rights:r for readw for writex (or s) for execute (programs) or open (directories)

chmod : change modechmod g+w test.hs gives (+) the group (g) the write access (w)

u/g/o/a [+/-/=] [r/w/x]


Software

Outline



3 Software

4 The editor

5 Haskell


Software

Running other software

Any program can be launched from the command lineSome create their own window:

firefox &emacs &

Some run in the terminal and shell:man manvim myfile.hsfind . -name myexercise.hs -print


Software

Background and Foreground

What does the ampersand mean?Starting a program in the foreground: emacsStarting a program in the background: emacs &Only have one foreground process

and then, it cannot do anything elseWhen a program runs in the background,

the shell continues doing other things (i.e. new prompt)

Programs running in the terminal window needs the foreground


Software

Moving to the background

Special keys in the terminal onlyInterrupt: Ctrl-C (terminates the program)Suspend: Ctrl-Z (program stops, but can rewake)

Rewaking a process:Background: bgForeground: fgList jobs: jobsbg %jobno.

If you forget the ampersand: Ctrl-Z + bg

If you suspend a terminal program: fg


Software

Processes and kill

List processes:Current terminal: psAll: ps xgNote PID (process ID) to the left

Kill a process:Terminate: kill PID or kill %jobno.Kill: kill -KILL (and PID or job no.)

Dynamic process list: topshows resource expenditure

If a process malfunctions, you can kill it.


The editor

Outline



3 Software

4 The editor

5 Haskell


The editor

The editor

The editor is our most important toolEdit plain text files

in Haskell, Java, C, English, HTMLno special formatting (i.e. not a word-processor)

Use your favourite editoremacs, vim, pico, etc.


The editor

emacs

Straight forward for most usersUni-modal editorRuns in a separate window (with menus etc.)

It is a powerful toolAdvanced editingMacrosPlugins for email, news, etc

Advanced features are hard to learn and few use them


The editor

vim

Bimodal editorAlien for many usersMany commands at few key strokes

Command modeall keys are used for commands (movement, replace, etc)

Insert modeTyped text is added to the file

Runs on the command line: very fast


The editor

Basics in vim/vi

Start it: vim filename.hs

Command mode→ Insert mode: i, I, a, A, o, OInsert mode→ Command mode: EscColon commands are followed by return:

Quit: :q (:q! forced)Save and quit: :wqSearch and replace: :%s/old/new/g

Delete: x, X (character), dd (line)Replace: r (character), R (several character (insert mode))See a full tutorial on the web page.


The editor

Starting the editor on a file

A file can be given on the command linevi myfile.hs starts vi on the given file

Editors running in the terminal (vi, pico)are almost always started this way

You can also run emacs myfile.hsbut many prefer loading the files from the menus

because emacs is heavy, subsequent files should be loaded fromthe menu

quitting and restarting is impractical

vi users often prefer quit and restart


Haskell

Outline



3 Software

4 The editor

5 Haskell


Haskell

The interpreter

Start hugs on the command linedifferent prompt

Reads and interprets commands and expressions:load module.hs56+3*9-7"Hello World!""Hello" ++ ", " ++ "George"

You can use it straight away as a calculator... but you’d rather want to make your own module with advancedfeatures


Haskell

Expressions

An expression can beA single number: 42 or 12.4An operator with arguments: 12 + 4 or 54.2/3A function call: sqrt 12 (

√12)

When you type an expresion,it is evaluated, i.e. functions and operators are applied to produce aresultthe result is displayed if possible


Haskell

What is a function?

x1 function y1

View the function as a machineA value x1 comes inA value y1 is returned

Using a function, it is a black boxYou don’t know what happens insideYou see only the output y1.


Haskell

For example?

2 sqrt (√ ) 1.4142135623731

Main> sqrt 21.4142135623731Main>


Haskell

Functional programming

Functions are the fundamental building block in functionalprogrammingContrary to imperative programming (e.g. Java)

... where data objects are the building blocksDuring the module we will see how

functions return functionsfunctions are arguments to functions


Haskell

Haskell ScriptsDefining functions and constants

Haskell scripts: e.g. demo.hsEdit scripts using an editor (vim, emacs)Load the script in hugs using :load

The script contains definitionsnot expressions

Note that you cannot make definitions on the command lineonly expressions


Haskell

Definitions and declarations

Declaration: myName :: String

Definition: myName = "George"This is not similar to Java

= denotes equality – it is a universal truthin Java = denotes an assignment

and myName could then be reassigned

a definition is done once and for all


Haskell

Function definitions

treble :: Int -> Int

treble x = 3*x

Hugs> :load 01intro/demo.hsMain> myName"George"Main> treble 515Main> treble 4.3ERROR - Cannot infer instance

*** Instance : Fractional Int

*** Expression : treble 4.3

Main>


Haskell

Lists

myList :: [Int]

myList = [12, 15, 24, 14, 12, 6]

Standard functions and operators exist for lists

Main> myList!!224Main> reverse myList[6,12,14,24,15,12]Main> drop 4 myList[12,6]Main> drop (2*2) (reverse myList)[15,12]Main>


Haskell

Naming requirementsHaskell is case sensitive

Haskell is case sensitivemyList and mylist are different variables

Case gives information about the identifierall variables start with lower-case : myListall type identifiers start with upper-case : Int

If you forget this, you will get confusing error messages...


Haskell

The layout rule

How does the interpreter know where a definition ends?In Java this is marked by a semicolon (;)Haskell uses layout, i.e. indentation

fun x y =a long

and winding expressionin x

+ yfun2 x y= another function in x and y

lines with extra indentation continues the definitionany text in the same column or to the left marks a new definition


Haskell

Good practice

Use indentation consistentlyTry to make it easy to read

max x y| x > y = x| otherwise = y

Align the guards (|) and the equal signs (=)Layout is mainly a question of personal style

... but please think of the reader


Haskell

Comments

Good practice: comment your codeComment starts with - and ends at the end of line

- the compiler ignore comments like this

Comments are for human readersexplain your codemake it easy to understand


Haskell

Summary

Quick overview of basic features and tools neededYou will have to test it all on the computer

lab sessionsand in your own time

This week’s material is basicyou have to master it


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