(add simple code for reading a file)
Revision as of 16:51, 25 February 2007
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
1.1 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
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 can be read as input using getLine.
Prelude> getLine Foo bar baz "Foo bar baz"
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 of strings is done with the `++' operator.
Prelude> "foo" ++ "bar" "foobar"
4 NumbersNumbers in Haskell can be of the type
4.1 Random numbers
5 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
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
7 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)
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
9.1 Simple IOUsing
A program to sum up numbers:
A program that adds line numbers to each line:
numberLines = unlines . zipWith combine [1..] . lines
where combine lineNumber text = concat [show lineNumber, " ", text]
9.2 Reading from files
The System.IO library contains the functions needed for file IO. The program below displays the contents of the file c:\test.txt.
main = do
h <- openFile "c:\\test.txt" ReadMode
contents <- hGetContents h
The same program, with some higher-lever functions:
contents <- readFile "c:\\test.txt"