Haskell: Guards

Guards are used to write programs effectively and cleanly. For example, following is the formula to calculate factorial of a number.

Above formula can be written in Haskell like below.

factorial.hs

factorial n
    | (n <= 0)  = 1
    | otherwise = (factorial (n-1)) * n

Let me explain, what I am doing here.

a.   ‘factorial n’ define a function which takes one argument ‘n’.

b.   Instead of defining function with equal sign I written alternatives using guards. Guards begin with | character, After the | (pipe) character we put an expression which evaluates to a boolean, which is followed by the rest of the definition. The function only uses the equals sign and the right-hand side from a line if the predicate evaluates to True.

c.     The otherwise case is used when none of the preceding predicates evaluate to True. In this case if n > 0, then other wise is executed.

Prelude> :load factorial.hs
[1 of 1] Compiling Main             ( factorial.hs, interpreted )
Ok, modules loaded: Main.
*Main> 
*Main> factorial 5
120
*Main> factorial 10
3628800
*Main> 
*Main> factorial 100
93326215443944152681699238856266700490715968264381621468592963895217599993229915608941463976156518286253697920827223758251185210916864000000000000000000000000
*Main> 
*Main> factorial 1000
402387260077093773543702433923003985719374864210714632543799910429938512398629020592044208486969404800479988610197196058631666872994808558901323829669944590997424504087073759918823627727188732519779505950995276120874975462497043601418278094646496291056393887437886487337119181045825783647849977012476632889835955735432513185323958463075557409114262417474349347553428646576611667797396668820291207379143853719588249808126867838374559731746136085379534524221586593201928090878297308431392844403281231558611036976801357304216168747609675871348312025478589320767169132448426236131412508780208000261683151027341827977704784635868170164365024153691398281264810213092761244896359928705114964975419909342221566832572080821333186116811553615836546984046708975602900950537616475847728421889679646244945160765353408198901385442487984959953319101723355556602139450399736280750137837615307127761926849034352625200015888535147331611702103968175921510907788019393178114194545257223865541461062892187960223838971476088506276862967146674697562911234082439208160153780889893964518263243671616762179168909779911903754031274622289988005195444414282012187361745992642956581746628302955570299024324153181617210465832036786906117260158783520751516284225540265170483304226143974286933061690897968482590125458327168226458066526769958652682272807075781391858178889652208164348344825993266043367660176999612831860788386150279465955131156552036093988180612138558600301435694527224206344631797460594682573103790084024432438465657245014402821885252470935190620929023136493273497565513958720559654228749774011413346962715422845862377387538230483865688976461927383814900140767310446640259899490222221765904339901886018566526485061799702356193897017860040811889729918311021171229845901641921068884387121855646124960798722908519296819372388642614839657382291123125024186649353143970137428531926649875337218940694281434118520158014123344828015051399694290153483077644569099073152433278288269864602789864321139083506217095002597389863554277196742822248757586765752344220207573630569498825087968928162753848863396909959826280956121450994871701244516461260379029309120889086942028510640182154399457156805941872748998094254742173582401063677404595741785160829230135358081840096996372524230560855903700624271243416909004153690105933983835777939410970027753472000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
*Main> 

a. Write Fibonacci series using guards

The Fibonacci Sequence is the series of numbers: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, ...

The next number is found by adding up the two numbers before it.    

fibonacci.hs

fibonacci n
    | (n == 0)  = 0
    | (n == 1)  = 1
    | otherwise = fibonacci (n-1) + fibonacci (n-2)

*Main> :load fibonacci.hs
[1 of 1] Compiling Main             ( fibonacci.hs, interpreted )
Ok, modules loaded: Main.
*Main> 
*Main> fibonacci 0
0
*Main> fibonacci 1
1
*Main> fibonacci 2
1
*Main> fibonacci 3
2
*Main> fibonacci 4
3
*Main> fibonacci 5
5
*Main> fibonacci 6
8
*Main> fibonacci 7
13

Previous                                                 Next                                                 Home