Cookbook
We need to start a GOOD (aka, not a PLEAC clone) Haskell cookbook. This page is based on the Scheme Cookbook at http://schemecookbook.org/Cookbook/WebHome
GHCi/Hugs
GHCi Interaction
To start GHCi from a command prompt, simply type `ghci'
$ ghci
___ ___ _
/ _ \ /\ /\/ __(_)
/ /_\// /_/ / / | | GHC Interactive, version 6.6, for Haskell 98.
/ /_\\/ __ / /___| | http://www.haskell.org/ghc/
\____/\/ /_/\____/|_| Type :? for help.
Loading package base ... linking ... done.
Prelude>
Prelude is the "base" library of Haskell.
To create variables at the GHCi prompt, use `let'
Prelude> let x = 5
Prelude> x
5
Prelude> let y = 3
Prelude> y
3
Prelude> x + y
8
Types
To check the type of an expression or function, use the command `:t'
Prelude> :t x
x :: Integer
Prelude> :t y
y :: Integer
Haskell has the following types defined in the Standard Prelude.
Int -- bounded, word-sized integers
Integer -- unbounded integers
Double -- floating point values
Char -- characters
String -- strings
() -- the unit type
Bool -- booleans
[a] -- lists
(a,b) -- tuples / product types
Either a b -- sum types
Maybe a -- optional values
Strings
Input
Strings can be read as input using getLine.
Prelude> getLine
Foo bar baz
"Foo bar baz"
Output
Strings can be output in a number of different ways.
Prelude> putStr "Foo"
FooPrelude>
As you can see, putStr does not include the newline character `\n'. We can either use putStr like this:
Prelude> putStr "Foo\n"
Foo
Or use putStrLn, which is already in the Standard Prelude
Prelude> putStrLn "Foo"
Foo
We can also use print to print a string, including the quotation marks.
Prelude> print "Foo"
"Foo"
Concatenation
Concatenation of strings is done with the `++' operator.
Prelude> "foo" ++ "bar"
"foobar"
Numbers
Numbers in Haskell can be of the type Int, Integer, Float, Double, or Rational.
Random Numbers
Dates and Time
Use System.Time.getClockTime to get a properly formatted date stamp.
Prelude> System.Time.getClockTime
Wed Feb 21 20:05:35 CST 2007
Lists
Haskell has all of the general list manipulation functions.
Prelude> head [1,2,3]
1
Prelude> tail [1,2,3]
[2,3]
Prelude> length [1,2,3]
3
Pattern Matching
Haskell does implicit pattern matching.
A good example of pattern matching is done in the fact function for finding a factorial.
fact :: Integer -> Integer
fact 0 = 1
fact n = n * fact (n - 1)
In this function, fact :: Integer -> Integer is the functions type definition.
The next line, fact 0 = 1 is a pattern match, so when the argument to the function fact is 0, the return value is 1.
The 3rd and final line of this function is another pattern match, which says that, whatever number was entered as the argument, is multiplied by the factorial of that number, minus 1. Notice this function is recursive.
Pattern matching in Haskell evaluates the patterns in the order they are written, so fact 0 = 1 is evaluated before fact n = n * fact (n - 1).