software/haskell
Haskell
Structure
Haskell programs consist of monads, actions, modules, ???
Basic Syntax
A “type declaration”:
main :: IO ()
An action definition. Note that whitespace matters; the block extends to all lines indented to the same position as the first non-whitespace after the do
:
main = do
stuff1
stuff2
Lists
Lists in haskell are homogenous: all elements must of the same type. They are linked lists, so cons-ing on the front is cheap and concatonating on the end can be expensive.
Use ++ to concatonate two lists together:
['a','b','c'] ++ ['d','e','f']
Use : to cons (prepend) a single element:
0:[1,2,3,4,5,6]
Use !! to pull out an element by index (zero indexed):
['c','a','t'] !! 1
Strings are lists of characters: “baby” is equivalent to [‘b’,‘a’,‘b’,‘y’], which is equivalent to ‘b’:‘a’:‘b’:‘y’:[]. The empty set is [] and is distinct from [[]].
A couple functions help; ‘head’ is like ‘car’ and gives the first element, ‘tail’ is like ‘cdr’ and gives everything except the first element, ‘last’ gives the last non-empty element, and ‘init’ gives everything except the ‘last’. ‘length’ gives the number of elements, ‘null’ is a test to see if this is the empty list,
Compilation
By default the “main” action of the “Main” module is the action that is executed when a compiled haskell program is run by the operating system; this means that most haskell programs need to define these components.
The ghc
(Glasgow Haskell Compiler) is the most popular. To compile and execute a simple one file haskell program you will do something like:
ghc -o hello helloworld.hs
./hello