Discount/Zero Curve Construction in F# – Part 1 (The End)

I wanted to learn a little bit about F# by implementing something more interesting than the obligatory fractals or Fibonacci sequences, so I thought I’d see what yield curve construction would look like, and how much less code would be needed versus a standard C/C++ implementation. The simplicity with which it’s possible to tail recurse over lists in F# lends itself well to the bootstrapping techniques employed for constructing a zero-coupon rate curve.

F# seems to read better from right to left and from bottom to top, so we’ll start with what might be the final step in building an interest rate discount curve, and work back from there:

let curve = [ (curveDate, 1.0) ]
            |> bootstrap spotPoints 
            |> bootstrapCash spotDate cashPoints
            |> bootstrapFutures futuresStartDate futuresPoints
            |> bootstrapSwaps spotDate USD swapPoints
            |> Seq.sortBy (fun (d, _) -> d)
This is a forward pipe, whereby the result of one expression or function in the pipeline is silently passed as the last argument to the next function, allowing composition in stages, but still quite concisely. We start with today’s money (each dollar of which is obviously worth one dollar today), as a tuple (Date * double), and separately bootstrap the short cash up to spot (typically 2 days from today in the US dollar market), longer cash out to a few months, several Eurodollar futures and some dollar swaps, and finally sort the sequence chronologically. The result is an ordered sequence of Date * double tuples that represent the full discount curve:
image image


What goes in?

Let’s look at what feeds the process. The curve date would typically be today’s date:

let curveDate = Date.Today
Spot, in the US market is usually two business days. We’ll get to the details of business day rolling conventions later on, but for now:
let spotDate = rollBy 2 RollRule.Following USD curveDate

A collection of quotes would be something like the list below. For illustration purposes we’ll use cash deposits out to three months, a small range of futures and swaps for the rest of the curve out to thirty years:

let quotes = [ (Overnight, 0.045);
               (TomorrowNext, 0.045);
               (Cash (tenor "1W"), 0.0462);
               (Cash (tenor "2W"), 0.0464);
               (Cash (tenor "3W"), 0.0465);
               (Cash (tenor "1M"), 0.0467);
               (Cash (tenor "3M"), 0.0493);
               (Futures (contract "Jun2009"), 95.150);
               (Futures (contract "Sep2009"), 95.595);
               (Futures (contract "Dec2009"), 95.795);
               (Futures (contract "Mar2010"), 95.900);
               (Futures (contract "Jun2010"), 95.910);
               (Swap (tenor "2Y"), 0.04404);
               (Swap (tenor "3Y"), 0.04474);
               (Swap (tenor "4Y"), 0.04580);
               (Swap (tenor "5Y"), 0.04686);
               (Swap (tenor "6Y"), 0.04772);
               (Swap (tenor "7Y"), 0.04857);
               (Swap (tenor "8Y"), 0.04924);
               (Swap (tenor "9Y"), 0.04983);
               (Swap (tenor "10Y"), 0.0504);
               (Swap (tenor "12Y"), 0.05119);
               (Swap (tenor "15Y"), 0.05201);
               (Swap (tenor "20Y"), 0.05276);
               (Swap (tenor "25Y"), 0.05294);
               (Swap (tenor "30Y"), 0.05306) ]


All of these quotes are tuples, with a second value of type float, representing the market quote itself. The type of the first value in the tuple we make a discriminated union, to distinguish the different types of instrument:

type QuoteType =
| Overnight                     // the overnight rate (one day period)
| TomorrowNext                  // the one day period starting "tomorrow"
| Cash of Tenor                 // cash deposit period in days, weeks, months
| Futures of FuturesContract    // year and month of futures contract expiry
| Swap of Tenor                 // swap period in years

A futures contract we can model simply as a date, and a tenor is generically a combination of year, month and/or day offsets:
type Date = System.DateTime
type FuturesContract = Date
type Tenor = { years:int; months:int; days:int }
Date and FuturesContract amount to little more than aliases for the Framework Class Library type DateTime, providing us with a shorthand notation, or clarifying the code. To that end we also throw in some helper functions for creating instances by parsing text. For tenors we use a regular expression match that should cover all the bases:
let date d = System.DateTime.Parse(d)
let contract d = date d
let tenor t =
    let regex s = new Regex(s)
    let pattern = regex ("(?<weeks>[0-9]+)W" + 
                         "|(?<years>[0-9]+)Y(?<months>[0-9]+)M(?<days>[0-9]+)D" +
                         "|(?<years>[0-9]+)Y(?<months>[0-9]+)M" + 
                         "|(?<months>[0-9]+)M(?<days>[0-9]+)D" +
                         "|(?<years>[0-9]+)Y" +
                         "|(?<months>[0-9]+)M" +
    let m = pattern.Match(t)
    if m.Success then
        { new Tenor with 
            years = (if m.Groups.["years"].Success then int m.Groups.["years"].Value else 0) and 
            months = (if m.Groups.["months"].Success then int m.Groups.["months"].Value else 0) and 
            days = (if m.Groups.["days"].Success then int m.Groups.["days"].Value 
                    else if m.Groups.["weeks"].Success then int m.Groups.["weeks"].Value * 7 
                    else 0) }
        failwith "Invalid tenor format. Valid formats include 1Y 3M 7D 2W 1Y6M, etc"
In part 2 we’ll look into the details of preparing the raw quotes for bootstrapping, which will include converting quotes into dates and rolling those dates to account for weekends and holidays.

To Begin With…

A few weeks ago at the Wintellect Devscovery conference in New York I was struck by a trio of coincidental insights. The first was from Scott Hanselman’s keynote, during which he suggested that there’s really very little reason why every developer shouldn’t have a personal blog. The second came from John Robbins’ session on debugging, where he reminded us that paper, pencil and our own brains are the best debugging tools we have.

I’ve gone through several methods of “thought management” over the years, ranging from the old Black n’ Red notebook to Microsoft OneNote. A while back one company I worked for required engineers (programming was engineering back then) to maintain a daily log of work and notes in an A4 Black n’ Red, and those books were ultimately the property of the company – how they expected to be able to assimilate or search that information I have no idea.

I like OneNote a lot, but it suffers slightly from the classic “fat client” problem, in that OneNote itself is an installed application that works primarily with local notebook files. It’s possible to put OneNote notebooks on a network share or a Live Mesh folder for sharing outside your LAN, but of course even then you need to have OneNote and/or Live Mesh installed on every machine from which you plan to access your notes. No kiosks need apply. And if you’d like to publish some of your more lucid thoughts…

So now, since I have some personal projects gathering source control dust, I’m going to try a personal blog for my notes. It’ll mean I’ll need to write up my notes a lot more coherently than I usually do, but as Robbins suggested for my third insightful reminder from that conference, oftentimes you solve a particularly gnarly problem after the first ten seconds of trying to explain it to someone else.

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