Text.RomanNumerals

Parsing and pretty printing of Roman numerals.

This module provides functions for parsing and pretty printing Roman numerals. Because the notation of Roman numerals has varied through the centuries this package allows for some customisation using a configuration that is passed to the conversion functions. Exceptions are dealt with by wrapping the results of conversions in the error monad.

Types

data (Ord n, Num n) => NumeralConfig n

A configuration with which the convertTo and convertFrom functions can be parameterized.

Constructors

NC

Fields

ncMax :: n

The largest value that can be represented using this configuration.

ncZero :: Maybe String

Symbol to represent the value 0. The Romans did not have a symbol for zero. If set to Nothing a convertFrom 0 will throw an error.

ncTable :: [(String, n)]

A table of symbols and their numerical values. The table must be ordered with the largest symbols appearing first. If any symbol is the empty string then convertFrom will be undefined. If any symbol in this table is associated with the value 0 both the convertTo and convertFrom function will be undefined.

Sample configurations

modernRoman :: (Ord n, Num n) => NumeralConfig n

Configuration for Roman numerals as they are commenly used today. The value 0 is represented by the empty string. It can be interpreted as not writing down a number. This configuration is limited to the range [1..3999]. Larger numbers can be represented using Roman numerals but you will need notations that are hard or impossible to express using strings.

simpleRoman :: (Ord n, Num n) => NumeralConfig n

Configuration for Roman numerals that do not use the rule that a lower rank symbol can be placed before a higher rank symbol to denote the difference between them. Thus a numeral like IV will not be accepted or generated by this configuration.

Pretty printing

convertTo :: (Ord n, Num n, MonadError String m) => NumeralConfig n -> n -> m String

Converts a number to a Roman numeral according to the given configuration. Numbers which are out of bounds will cause exceptions to be thrown. An exception will also be raised if no representation is possible with the given configuration. If the value of any symbol in the configuration is equal to 0 or a symbol is the empty string this function is undefined.

unsafeConvertTo :: (Ord n, Num n) => NumeralConfig n -> n -> String

Like convertTo, but exceptions are promoted to errors.

toRoman :: (Ord n, Num n, MonadError String m) => n -> m String

Converts a number to a modern Roman numeral. See convertTo for possible exceptions.

unsafeToRoman :: (Ord n, Num n) => n -> String

Like toRoman, but exceptions are promoted to errors.

Parsing

convertFrom :: (Ord n, Num n, MonadError String m) => NumeralConfig n -> String -> m n

Parses a string as a Roman numeral according to the given configuration. An exception will be raised if the input is not a valid numeral.

fromRoman :: (Ord n, Num n, MonadError String m) => String -> m n

Parses a string as a modern Roman numeral. See convertFrom for possible exceptions.

unsafeConvertFrom :: (Ord n, Num n) => NumeralConfig n -> String -> n

Like convertFrom, but exceptions are promoted to errors.

unsafeFromRoman :: (Ord n, Num n) => String -> n

Like fromRoman, but exceptions are promoted to errors.