Text.Numeral.Roman

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.

Example:

  > toRoman 1729 ∷ String
  "MDCCXXIX"

  > fromRoman "MDCCXXIX" ∷ Either String Integer
  Right 1729

  > convertTo simpleRoman 1729 ∷ String
  "MDCCXXVIIII"

  > fromRoman "Bla" ∷ Either String Integer
  Left "Roman.convertFrom: can't parse"

Types

data NumeralConfig s n

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

mkNumConfig

Arguments

:: (Ord n, Num n)

=> s

Symbol for zero

-> s

Symbol for one

-> [(s, n)]

Symbol-value table.

-> NumeralConfig s n

Smart constructor for a NumeralConfig.

Pretty printing

convertTo :: Monoid s => Ord n => Num n => NumeralConfig s n -> n -> s

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.

Parsing

class StripPrefix s where

Class used to overload the input of the parsing functions.

Methods

spNull :: s -> Bool

spStripPrefix :: s -> s -> Maybe s

Instances

StripPrefix ByteString

Eq α => StripPrefix [α]

convertFrom :: (Monoid s, StripPrefix s, Eq s, Ord n, Num n, MonadError String m) => NumeralConfig s n -> s -> m n

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

Default Configurations

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

Configuration for Roman numerals as they are commonly used today. The value 0 is represented by the empty string. It can be interpreted as not writing down a number. This configuration is practically limited to the range [1..3999]. Smaller numbers will result in empty string. Larger numbers will result in repeated use of the 'M' symbol.

simpleRoman :: (IsString s, Ord n, Num n) => NumeralConfig s 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. This configuration is practically limited to the range [1..3999]. Larger numbers will result in repeated use of the 'M' symbol.

Utility

toRoman :: (IsString s, Monoid s, Ord n, Num n) => n -> s

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

fromRoman :: (IsString s, Monoid s, StripPrefix s, Eq s, Ord n, Num n, MonadError String m) => s -> m n

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