map = fmap

 empty = mempty

 (++) = mappend

 concat = mconcat

 intercalate = mconcat .: intersperse

Compute the sum of a finite list of numbers.

Compute the product of a finite list of numbers.

Convert a value to readable Text

Parse Text to a value

The readIO function is similar to read except that it signals parse failure to the IO monad instead of terminating the program.

Read a file and return the contents of the file as Text. The entire file is read strictly.

Write Text to a file. The file is truncated to zero length before writing begins.

Write Text to the end of a file.

O(n) Breaks a Text up into a list of Texts at newline Chars. The resulting strings do not contain newlines.

O(n) Breaks a Text up into a list of words, delimited by Chars representing white space.

O(n) Joins lines, after appending a terminating newline to each.

O(n) Joins words using single space characters.

Write a string to stdout.

Read a single line of user input from stdin.

Lazily read all user input on stdin as a single string.

The interact function takes a function of type Text -> Text as its argument. The entire input from the standard input device is passed (lazily) to this function as its argument, and the resulting string is output on the standard output device.

gcd x y is the non-negative factor of both x and y of which every common factor of x and y is also a factor; for example gcd 4 2 = 2, gcd (-4) 6 = 2, gcd 0 4 = 4. gcd 0 0 = 0. (That is, the common divisor that is "greatest" in the divisibility preordering.)

Note: Since for signed fixed-width integer types, abs minBound < 0, the result may be negative if one of the arguments is minBound (and necessarily is if the other is 0 or minBound) for such types.

lcm x y is the smallest positive integer that both x and y divide.

A String is a list of characters. String constants in Haskell are values of type String.

The shows functions return a function that prepends the output String to an existing String. This allows constant-time concatenation of results using function composition.

Convert a value to a readable String.

showsPrec should satisfy the law

 showsPrec d x r ++ s  ==  showsPrec d x (r ++ s)

Derived instances of Read and Show satisfy the following:

• (x,"") is an element of (readsPrec d (showsPrec d x "")).

That is, readsPrec parses the string produced by showsPrec, and delivers the value that showsPrec started with.

The method showList is provided to allow the programmer to give a specialised way of showing lists of values. For example, this is used by the predefined Show instance of the Char type, where values of type String should be shown in double quotes, rather than between square brackets.

equivalent to showsPrec with a precedence of 0.

utility function converting a Char to a show function that simply prepends the character unchanged.

utility function converting a String to a show function that simply prepends the string unchanged.

utility function that surrounds the inner show function with parentheses when the Bool parameter is True.

A parser for a type a, represented as a function that takes a String and returns a list of possible parses as (a,String) pairs.

Note that this kind of backtracking parser is very inefficient; reading a large structure may be quite slow (cf ReadP).

attempts to parse a value from the front of the string, returning a list of (parsed value, remaining string) pairs. If there is no successful parse, the returned list is empty.

Derived instances of Read and Show satisfy the following:

• (x,"") is an element of (readsPrec d (showsPrec d x "")).

That is, readsPrec parses the string produced by showsPrec, and delivers the value that showsPrec started with.

The method readList is provided to allow the programmer to give a specialised way of parsing lists of values. For example, this is used by the predefined Read instance of the Char type, where values of type String should be are expected to use double quotes, rather than square brackets.

equivalent to readsPrec with a precedence of 0.

readParen True p parses what p parses, but surrounded with parentheses.

readParen False p parses what p parses, but optionally surrounded with parentheses.

The lex function reads a single lexeme from the input, discarding initial white space, and returning the characters that constitute the lexeme. If the input string contains only white space, lex returns a single successful `lexeme' consisting of the empty string. (Thus lex "" = [("","")].) If there is no legal lexeme at the beginning of the input string, lex fails (i.e. returns []).

This lexer is not completely faithful to the Haskell lexical syntax in the following respects:

• Qualified names are not handled properly
• Octal and hexadecimal numerics are not recognized as a single token
• Comments are not treated properly

Write a character to the standard output device (same as hPutChar stdout).

Read a character from the standard input device (same as hGetChar stdin).

The readLn function combines getLine and readIO.

The Haskell 98 type for exceptions in the IO monad. Any I/O operation may raise an IOError instead of returning a result. For a more general type of exception, including also those that arise in pure code, see Control.Exception.Exception.

In Haskell 98, this is an opaque type.

Raise an IOError in the IO monad.

Construct an IOError value with a string describing the error. The fail method of the IO instance of the Monad class raises a userError, thus:

 instance Monad IO where 
   ...
   fail s = ioError (userError s)