currency-convert

Property	Value
Module	Data.Currency.Convert
Description	Typesafe currency coversion using exchange rates from various sources.
Copyright	© Tuomas Laakkonen 2016
License	BSD3
Maintainer	[email protected]
Stability	Experimental
Portability	Non-portable (GHC extensions)

This module allows values of currency to be converted from one currency to another using exchange rates from various sources.

Basic Operation

The basic operation for this module is as follows:

First, get a conversion function (with getDefaultConverter or getConverter):

    >>> Converter convert <- getDefaultConverter

Then, construct some value of currency using the convenience functions, and convert it using the convert function you just got:

    >>> convert (usd 100) :: EUR
    90.0 eur

The result type of convert defines what currency the value will be converted into.

Currency implements Num, Fractional and Real so the usual arithmetic operations are defined, and its value can be extracted with toRational.

Defining New Currencies

New currencies can be defined very simply:

    type NEW = Currency "new"
    new :: Real a => a -> NEW
    new = unsafeCoerceCurrency . fromRational . toRational

For some hypothetical currency new. These can be used like any other currency.

Defining New Rate Providers

A method for providing new sources of currency conversion rates is also provided:

Simply create a RateProvider. The arguments to RateProvider are as follows:

	RateProvider (name :: String) (getRates :: IO RateDict)

Where RateDict is just a list of (name :: String, rate :: Double) where rate is the conversion rate between the specified currency and the Euro.

Using Rate Providers

Different rate providers can be used by providing them as an argument to getConverter. You can also chain these using the combinator (<|-|>) which tries one rate provider before trying another if that fails (e.g online api followed by a local cache if that fails followed by a hardcoded dictionary).