Haskell is a pure functional language; no state information is kept. In other words, the state of a computation can be fully described by the state of the stack.
Gofer shares the same core language as Haskell. Each language has some features the other lacks, but this paper discusses only the common core. For convenience we use the name "Haskell" throughout this paper, rather than repeatedly saying "Haskell and Gofer."
Haskell is a particularly enjoyable language in which to program, as the language does so much to make it easy for the programmer. A lot of structural syntax, such as parentheses and semicolons, simply aren't needed. Powerful operators allow for concise, yet still readable, programs. Although the language is strongly typed, the programmer hardly ever needs to specify types--Haskell figures out types for itself. Functions are first-class objects, and infinite data structures are supported. Haskell is more human-oriented and less computer-oriented than most programming languages and, as a result, is less efficient; it therefore has little practical application.
Haskell has an underlying uniform representation. But for convenience, it has been extended with syntactic sugar to provide familiar-looking representations for arithmetic, strings, lists, and the like.
There are two kinds of comment in Haskell:
-- Anything after a double hyphen, up to the end of the line.
{- Comments may be enclosed in braces with hyphens. {- These comments can be nested -} . -}
There are four simple types:
Primitive type |
Representative values |
Int |
-5, 0, 5 |
Float |
3.14159 |
Char |
'a', '\n', '\\', '\'', '\97', '\o77', '\xFF', '\DEL' |
Bool |
True, False |
A familiar set of functions and operators is provided. In the following table, the notation Char -> Int means a function that takes a character argument and produces an integer result; the notation Int -> Int -> Int means a function that takes two integer arguments and produces an integer result. (The last type in the chain is always the result.) The reason for this notation for types will be explained later.
Functions on integers:
Function |
Type |
Comment |
+ - * / ^ |
Int -> Int -> Int |
Infix add, subtract, multiply, divide, exponentiate |
even odd |
Int -> Bool |
Prefix tests for parity |
div quot |
Int -> Int -> Int |
div rounds down (same as /); |
gcd lcm |
Int -> Int -> Int |
greatest common divisor, least common multiple |
primIntToFloat |
Int -> Float |
convert to floating point |
signum |
Int -> Int |
-1, 0, or 1, according to whether the argument is negative, zero, or positive |
Functions on floating point numbers:
Function |
Type |
Comment |
+ - * / ^ |
Float -> Float -> Float |
Infix add, subtract, multiply, divide, exponentiate |
sin cos tan log exp sqrt log log10 |
Float -> Float |
Usual math functions |
pi |
Float |
3.1415926535 |
truncate |
Float -> Int |
Usual math functions |
Functions on characters:
Function |
Type |
Comment |
ord |
Char -> Int |
Convert to integer ASCII value |
chr |
Int -> Char |
Convert from ASCII value |
isPrint isSpace |
Int -> Bool |
Test for printable or nonprintable character |
isAscii isControl |
Int -> Bool |
Test for ASCII (?), control character |
isUpper isLower |
Int -> Bool |
Test for capital or lowercase letter |
isAlpha isDigit |
Int -> Bool |
Test for letter, test for decimal digit |
isAlphanum |
Int -> Bool |
Test for letter or digit |
Functions on Booleans:
Function |
Type |
Comment |
&& || |
Bool -> Bool -> Bool |
Infix and, infix or |
not |
Bool -> Bool |
Prefix not |
Polymorphic functions:
Function |
Type |
Comment |
< <= == /= >= > |
a -> a -> Bool |
Arguments must have same type |
show |
a -> String |
Convert almost anything to a string |
Haskell is an interactive language: you type an expression at the Haskell prompt, and Haskell prints the result of evaluating that expression. Thus, if you type 2+2 at the Haskell prompt, Haskell will respond with 4. The usual precedence rules apply, and parentheses may be used. For example, 2+3*4+5 means the same as 2+(3*4)+5. Integers and floating point may not be mixed in the same expression without explicit conversions.
Function definitions must be loaded from a file; they cannot be defined from the Haskell prompt. (This is a consequence of the fact that no state information is kept.) The user interacts with the system by making function calls. There is no "main program"--any function may be called.
In addition to evaluating expressions, the Haskell interpreter recognizes certain commands, all of which begin with a colon:
Command |
Meaning |
---|---|
:? |
Display a list of the available commands |
:l filename |
Load in definitions from the specified file |
:t expression |
Give the type of the expression as well as its value |
:q |
Quit |
To call a function, write the name of the function followed by its arguments. No additional punctuation is used. For example,
f x y
mod 103 10
Case is significant. Variable names and function names begin with a lowercase letter and may contain letters, digits, underscores, and apostrophes. Type names begin with a capital letter.
Binary infix operators (such as + and >) may be converted to prefix form by enclosing them in parentheses. Thus,
(+) 2 2 |
is the same as |
2 + 2 |
Binary prefix operators (such as mod) can be written in infix form by enclosing them in backquotes. Thus,
mod 100 3 |
is the same as |
100 `mod` 3 |
Haskell also contains lists, tuples, and functions.
Lists are written using brackets and commas, e.g. [1,2,4,8]. All the elements of a list must be of the same type; the type of a list is denoted [T], where T is the type of the elements. The list [a], where a is a variable, denotes a list whose elements are all of type a.
The empty list is denoted by []. Lists can be appended (concatenated) with the ++ operator.
A string is a list of characters, that is, it has type [Char]. For convenience, a string may be written with double quotes, e.g. "Haskell" is the same thing as ['H','a','s','k','e','l','l']. Because strings are lists, they may be concatenated with the ++ operator.
Functions on lists (and therefore strings):
Function |
Type |
Comment |
---|---|---|
: |
a -> [a] -> [a] |
(infix) Add an element to the front of the list |
++ |
[a] -> [a] -> [a] |
(infix) Append two lists |
!! |
[a] -> Int -> a |
(infix) Return element Int of the list, counting from zero |
head |
[a] -> a |
Return the first element of the list |
tail |
[a] -> [a] |
Return the list with the first element removed |
last |
[a] -> a |
Return the last element in the list |
init |
[a] -> [a] |
Return the list with the last element removed |
reverse |
[a] -> [a] |
Return the list with the elements in reverse order |
take |
[a] -> Int -> [a] |
Return the first Int elements of the list |
drop |
[a] -> Int -> [a] |
Return the list with the first Int elements removed |
nub |
[a] -> [a] |
Return the list with all duplicate elements removed |
elem notElem |
[a] -> a -> Bool |
Test for membership in the list |
length |
[a] -> Int |
The number of elements in the list |
concat |
[[a]] -> [a] |
Given a list of lists, concatenate all the lists into one list |
There are various shortcuts to writing lists. Haskell uses a technique called lazy evaluation: no value is ever computed until it is needed. Lazy evaluation allows Haskell to support infinite lists (and other infinite data structures). Arithmetic over infinite lists is supported, but some operations must be avoided, for example, it is a bad idea to ask for the last element of an infinite list.
Here are some list notations, with brief explanations and examples:
Notation |
Explanation |
Example |
---|---|---|
[a..b] |
The list of all values from a to b, inclusive |
[1..5] == [1,2,3,4,5] |
[a..] |
The list of all values equal to or larger than a |
[1..] == all positive integers |
[a, b..c] |
The list of values starting from a and stepping by (b-a), up to and possibly including c |
[1,3..10] == [1,3,5,7,9] |
[a, b..] |
The list of values starting from a and stepping by (b-a) |
[1, 3..] == all odd positive integers |
[expression_involving_x | x <- list] |
The set of all values of the expression involving x, where x is drawn from the list (this is called a list comprehension) |
[x*x | x <- [1..]] == the list of squares of positive integers |
[expression_involving_x_and_y | x <- list, y <- list] |
The set of all values of the expression involving x and y, where x is drawn from one list and y from the other (y varies faster) |
[[x,y] | x <- ['a'..'b'], y <- ['x'..'z']]
== |
[expression_involving_x | x <- list, condition_on_x] |
The set of all values of the expression involving x, where x is drawn from the list and meets the condition |
[x*x | x <- [1..10], even x] == |
[expression_involving_x_and_y | x <- list, condition_on_x, y <- list, condition_on_y] |
The set of all values of the expression involving x and y, where x is drawn from one list and meets one condition, while y is drawn from the other list and meets the other condition |
[x+y | x <- [1..5], even x, y <- [1..5], odd y] == [3, 5, 7, 5, 7, 9] |
As an example of the use of lazy evaluation, the infinite list [x*x | x <- [1..]] is the list of squares of positive integers.
Tuples are written using parentheses and commas, e.g. ("John",24,True). The elements of a tuple may be of different types. Two tuples have the same type if they have the same number of elements and those elements have the same types in the same order.
There are very few operations defined on tuples. If a tuple has exactly two elements, the functions fst and snd return the first and second elements, respectively. In general, you access the elements of a tuple by pattern matching, as explained below.
In Haskell, a function is a "first-class object," able to be used the same way other types are used (e.g. passed as arguments to functions).
Functions are defined using the = operator. For example,
averageOf2 x y = (x + y) / 2
This is actually syntactic sugar for the following:
averageOf2 = \x y -> (x + y) / 2
The expression on the right of the equals sign indicates an (anonymous) function of two arguments, x and y, with the body of the function following the arrow; this function is "assigned to" the variable on the left of the equals sign. (The backslash in this expression is pronounced "lambda.") Anonymous functions are used frequently in Haskell, usually surrounded by parentheses.
Haskell supports function "slices." For example, given the above definition, the expression (averageOf2 25) denotes the function that returns the average of 25 and its (single) argument. As a further example, you could define a Boolean function negative as
negative = (< 0)
(The parentheses are necessary.) Other examples of function slices are (2 *), (2 /), (/ 2), and (1 -). However, (- 1) does not work, because '-' is considered to be the unary minus.
Due to slicing, all Haskell functions could be thought of as having a single argument; for example, a function with type a -> b -> c -> d may be considered to be a function that takes a value of type a as an argument and returns as result a new function of type b -> c -> d, which in turn takes a value of type b as an argument and returns a new function of type c -> d, which takes a value of type c as an argument and returns a result of type d. Thus, the implicit parenthesization is: a -> b -> c -> d == a -> (b -> (c -> d))
Here are some functions that take functions as arguments:
Function |
Type |
Comment and example |
map |
(a->b) -> [a] -> [b] |
Applies the function to all elements of the list: |
filter |
(a->Bool) -> [a] -> [a] |
Returns the list of elements for which the function
returns True: |
iterate |
(a -> a) -> a -> [a] |
returns the list [x, f x, f f x, f f f x, ...]: |
foldl |
(a -> b -> a) -> a -> [b] -> a |
foldl f i x starts with the value i and
accumulates the value obtained by repeatedly applying
f to the current value and the next element in the
list: |
foldl1 |
(a -> a -> a) -> [a] -> a |
Same as foldl f (head x) (tail x) |
flip |
(a -> b -> c) -> b -> a -> c |
Return a function whose first two arguments are
reversed: |
(.) |
(a -> b) -> (c -> a) -> c -> b |
Compose two functions into a single function: |
span |
(a -> Bool) -> [a] -> ([a],[a]) |
Break the list into a tuple of two lists: all those at
the front of the list that satisfy the test, and the rest of
the list: |
break |
(a -> Bool) -> [a] -> ([a],[a]) |
Break the list into a tuple of two lists: all those at
the front of the list that fail the test, and the rest of
the list: |
Parameter transmission is by pattern matching; thus, a function definition may consist of two or more equations with differing formal parameters. For example, the factorial function may be defined as:
fact 0 = 1
fact n = n * fact (n - 1)
Equations will be tried in order; the pattern 0 in the first equation for fact will match only a call with a zero argument, while the pattern n in the second equation will match anything.
Here are the allowable patterns:
Type of pattern |
Examples |
Comment |
---|---|---|
A variable |
x |
Will match anything |
A constant |
5 'a' [] |
Will match only that value |
Wildcard |
_ |
"Don't care"--will match anything, but the matched value can't be used |
Tuples |
(x, y) |
Matches a pair--the component values can be referred to as x and y |
Fixed-length lists |
[x] [x,_,y] [] |
With the bracket notation, you can only match lists of the given length |
Variable-length lists |
(h:t) |
Matches a nonempty list whose head is h and whose tail is t |
As-patterns |
p@(x,y,z) p@(h:t) |
When the pattern after the @ matches something, the variable p matches the whole thing; e.g. if p@(h:t) matches [1,2,3], then h matches 1, t matches [2,3], and p matches [1,2,3] |
(n+k) patterns |
(m+1) |
Matches any value equal to or greater than k (k must be an integer); m is k less than the value matched |
Pattern matching allows you to write your own functions to extract the fields of tuples. For example, if you have tuples of the form ("Mary", 'f', 38), you might write extraction functions as follows:
name (x, _, _) = x
gender (_, x, _) = x
age (_, _, x) = x
A function consisting of multiple equations may have guards on each equation, as for example:
fact n | n == 0 = 1 | otherwise = n * fact (n - 1)
This is the same as
fact n = case n of 0 -> 1 n -> n * fact (n - 1)
Another way to write the factorial function is:
fact n = if n==0 then 1 else n * fact (n - 1)
You can introduce new variables with an expression of the form
let declarations in expression
for example:
fact n | n==0 = 1 | otherwise = let m = n - 1 in n * fact m
Another way to introduce local variables is with
function_definition where declarations
as for example in:
fact n | n==0 = 1 | otherwise = n * fact m where m = n - 1
Whereas let is an expression and may be used wherever an expression may be use, where is part of the syntax of a function declaration, and can only be used in this way. The scope of variables declared by where is the entire equation.
Haskell is a strongly-typed language, but you seldom need to tell Haskell what type your variables and functions are, because Haskell can almost always figure it out. It is a good idea to specify the types of functions, however, so that Haskell can inform you if a function doesn't have the type you intended. The operator :: means "has type," and can be used as follows:
fact :: Int -> Int
Indentation is sometimes important in Haskell.
An equation starts on a new line. Successive lines must be indented; when Haskell finds a line beginning in the same (or earlier) column as the start of the last equation, it takes this as the start of the next equation. Suggested indentation: start every equation in column 1, and indent all remaining lines of the equation. See the various definitions of fact above for examples.
The first declaration following let or where defines the starting column for that group of declarations; all additional declarations must begin in the same column.
latinize word = (snd pair) ++ (fst pair) ++ "ay" where vowel = flip elem "aeiou" consonant x = isLower x && not (vowel x) pair = span consonant word
The first printing character following the word of in a case statement defines the starting column for that group of cases; all additional cases must begin in the same column. One of the definitions of fact given above illustrates this layout.
Haskell is a rich language, and a number of important topics have not been discussed in this paper. Among these are irrefutable patterns, I/O, user-defined data types and type synonyms, polymorphism, type classes, overloading, contexts, and dictionaries.
The following definition of the quicksort algorithm shows just how nice Haskell can be (at least sometimes).
quicksort [] = [] quicksort (pivot:xs) = quicksort [x | x <- xs, x < pivot] ++ [pivot] ++ quicksort [x | x <- xs, x >= pivot]