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Making DSLs in F#

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16 Aug 2009CPOL9 min read 91.3K   492   59   14
Let's create a simple project estimation DSL using F#!



If you're like me, you are already fed up with people throwing the term ‘DSL’ around without showing a good example of how it’s done and where they are used – not to mention giving a decent, human-readable description of DSLs that doesn't allude to extraneous concepts (I'm talking about things like Oslo or MPS, mainly).

Okay, what on Earth is a DSL? A DSL is a way of defining domain-specific (you could say industry-specific, but it can be much narrower) logic using English instead of a programming language. The obvious benefit is that non-technical people can edit the DSL, without worrying about curly braces, semicolons, that sort of thing.

In this article, I'm going to show how to use the F# programming language to make a simple DSL that helps with software estimation. Just so you know, it’s a real-life example, with a more complex version of the DSL being used at our company. All right, let’s hit it!

Just to say upfront, this article doesn't present book-perfect F# code, the reason being that it doesn't really matter since F# code is our end-product. I can think of a dozen ways to improve the F# code presented, but, as I said, that’s not the purpose of this exercise.

Source Code

The attached code is a single .fs file, since I would probably get flogged for posting a VS2010 solution. I hope you know what to do with it. To run it, you need to have Project 2007 installed on your machine – otherwise it won’t work. Good luck!

Problem Statement

When someone wants software written, they typically contact a software development company with something called an RFP, short for “Request for Proposal”. Depending on the level of detail of this RFP, the code shop can either do a detailed estimate or give a ballpark figure. Barring cases of clients wanting dedicated teams or having vague requirements, the estimate the code shop makes is a fixed-price project timeline. Yeah, I know it doesn't sound particularly agile to people, but in situations where the prospective client has done lots of up-front design, it actually makes sense.

Anyway, someone has to do the estimate, i.e. partition the project into tasks, give them duration, assign resources (= people) to tasks, define milestones, et cetera. This sort of estimate can be done in a program such as Microsoft Project – a good choice in our case, since automating Office from an F# app is easy.

So why a DSL then? Well to be honest, you can probably do an estimate at the 10th of the time it takes to draw up the GANTT chart with all the reorderings and point-and-click mechanics of Project. Not only that, but typically you need to apply some sort of logic (i.e. use your brain, ugh!) to make sure the project plan is nice and balanced (I mean resource utilization and such). Having a DSL means you can optimize and autogenerate the plan. With a DSL, you can stick as much business logic into your planning procedure as you want – for example, if your company has a process database (this is CMMI Level 4-ish, btw), you could try validating the plan against empirical data.

Are you sold on the DSL idea for estimates? If not, here’s another boon: integration. You can integrate Project with other systems to get more value from existing data. For example, if you run Dynamics CRM, you can tweak resource pricing for a particular client in order to put better developers on the features they consider important. It all sounds very pompous and BI-ish, but that’s life of a code shop.

End Users

In most code shops, barring a few exceptions, estimates are done by project managers (PMs). These guys are sometimes techies, sometimes not, so you can't expect them to know programming. But you can certainly expect them to be able to work with a DSL and then press some magic key to generate a project plan or otherwise involve their DSL scribblings in an estimation BI scenario.

Continuous Process Improvement

At the risk of sounding cheeky, if you are constantly working on your (say) estimation DSL, improving it and tailoring it, it’s an excellent opportunity for improving your business processes. Think about it – as a code shop, you can use idle dev resources to improve the efficiency of your business. Is this great, or what? I think it is, anyway.

Choosing a Language

With the problem out of the way, let’s think of a solution. You can certainly make a free-form DSL language and write a parser, but it’s kind of tedious. A simpler solution is to use a programming language which looks so English (no cultural bias here – feel free to use Japanese or any other language) that the end user won’t know the difference. Of course, some language syntax will creep into the DSL, but its level varies.

Popular languages for DSLs include Boo (which Ayende is trying to make popular), Ruby and F#. Boo is extremely powerful, and for cases where you need metaprogramming support (not our scenario), it’s fantastic. Ruby I know next to nothing about, so no comment. Now, F# is a popular language, and a first-class citizen of .NET infrastructure (as of VS2010 especially). So we're going to look at making an infrastructure in which PMs can easily write project estimates.

Let me issue a small disclaimer: being a language geared towards immutability, F# looks a bit weird when used with highly mutable concepts, such as a project which can accumulate tasks or milestones. This weirdness can be easily compensated by writing the DSL data structures in C# and then using them from F#. However, for this example, I'll use F# exclusively.

First DSL Statement

I'll try to keep this simple. Let’s start by defining a project with a name and start date:

project "Write F# DSL Article" starts_on "16/8/2009"

The above is a completely legal F# statement. It’s basically a function call to a function called project which takes 3 parameters. The first parameter is a project name – no, you can't avoid the quotes here unless you want to parse stuff yourself. The second parameter is a dummy – a constant whose value is unimportant; its only purpose here is to make the spec readable and BDD-esque. You're welcome to expand the starts_on keyword into two separate parts, but I prefer not to overdo it, especially when there’s a risk of keywords creeping in. The third parameter is the project starting date as a string.

Believe it or not, the DSL uses OOP constructs in order to manage the constructed project. For example, we have a Project class which is our DSL representation of a project. It’s shown below. Skipping ahead a bit, make sure your types don't collide with other assemblies’ types. After all, Microsoft Project assemblies might just have a Project type too.

type Project() =
  [<DefaultValue>] val mutable Name : string
  [<DefaultValue>] val mutable Resources : Resource list
  [<DefaultValue>] val mutable StartDate : DateTime
  [<DefaultValue>] val mutable Groups : Group list

I did warn about the strange syntax, did I not? The above is F#’s way of making public fields. You'll notice also that I use list types instead of the System.Collections.Generic ones. It doesn't really matter what you use for the DSL so long as it works.

Our DSL will support just one project which will be at ‘global scope’, so to speak:

>let mutable my_project = new Project()

The naming convention is a bit ad hoc here save for the fact that, when we get to the end of the spec, we need a statement to actually do something and my_project is a nice identifier name for that action. But now, we can finally show the project statement from earlier.

let project name startskey start =
  my_project <- new Project()
  my_project.Name <- name
  my_project.Resources <- []
  my_project.Groups <- []
  my_project.StartDate <- DateTime.Parse(start)

There. I have probably revealed 90% of what DSL construction is like. You can close this article and go off exploring right now, since you already know how it all works. In the rest of this article, I'll be showing a few F# implementation details.

Handling Lists

Work in projects is done by resources (not very gratifying, is it?). A resource is a particular person (“John”) with a particular job title (“Junior DBA”) and a particular hourly rate ($65). A project keeps references to resources via a Resource list (see, F# is human-readable). Let’s look at the definition of Resource:

type Resource() =
  [<DefaultValue>] val mutable Name : string
  [<DefaultValue>] val mutable Position : string
  [<DefaultValue>] val mutable Rate : int

I'm abusing F# once again, but at least the structure is easy to work with. Now, a resource definition is also part of our DSL, and might look as follows:

>resource "Dmitri" isa "Project Manager" with_rate 140

The above statement employs the same trickery as project except that it acts on an already-existing global variable my_project:

let resource name isakey position ratekey rate =
  let r = new Resource()
  r.Name <- name
  r.Position <- position
  r.Rate <- rate
  my_project.Resources <- r :: my_project.Resources

Resources and all other lists we use end up being listed in reverse order. It’s not a problem though – we reverse them when the time comes. If you don't like it, use List<T> instead.

Referencing with Strings

The next concept of our DSL I want to introduct is a group of tasks. A group of tasks is typically done by one person to maintain, ahem, cognitive cohesion. We define a group as follows:

>group "Project Coordination" done_by "Dmitri"

To put things in context, let’s look at the Group class:

type Group() =
  [<DefaultValue>] val mutable Name : string
  [<DefaultValue>] val mutable Person : Resource
  [<DefaultValue>] val mutable Tasks : Task list

A group references a particular resource, which our DSL specifies only as a string. Problem? I don't think so:

let group name donebytoken resource =
  let g = new Group()
  g.Name <- name
  g.Person <- my_project.Resources |> List.find(fun f -> f.Name = resource)
  my_project.Groups <- g :: my_project.Groups

Notice how, unlike with LINQ, we don't have to call Single() after searching for the right resource.

Greater Fluency

Last but not least, we define tasks. Now, isn't it great when you can say, for example, the following:

task "PayPal Integration" takes 2 weeks

In fact, you can. This type of fluency is achieved by judiciously defining timespan constants so that their values are meaningful. For example:

let hours = 1
let hour = 1
let days = 2
let day = 2
let weeks = 3
let week = 3
let months = 4
let month = 4

The values do not matter so long as they are distinct. Now we can define a task…

type Task() =
  [<DefaultValue>] val mutable Name : string
  [<DefaultValue>] val mutable Duration : string

… and add it to the project:

let task name takestoken count timeunit =
  let t = new Task()
  t.Name <- name
  let dummy = 1 + count
  match timeunit with
  | 1 -> t.Duration <- String.Format("{0}h", count)
  | 2 -> t.Duration <- String.Format("{0}d", count)
  | 3 -> t.Duration <- String.Format("{0}wk", count)
  | 4 -> t.Duration <- String.Format("{0}mon", count)
  | _ -> raise(ArgumentException("only spans of hour(s), day(s), week(s) and month(s) are supported"))
  let g = List.hd my_project.Groups
  g.Tasks <- t :: g.Tasks

Notice that for each timespan, I slightly change the way the duration is phrased so that Project is capable of eating the spec. The dummy expression above tells F# that count is an integer – I could have defined it explicitly, of course, but I'm just too lazy. Oh, by the way, notice how easy it is for us to find the current (i.e., last) task. Because we're using F#, this task is actually first in the list, so we can just call List.hd.

Generating the Project

We've got everything and are ready to generate the project. The following (somewhat cheesy) command does it:

prepare my_project

Now, I'm about to show you the whole prepare definition, which uses the Project API to make the, umm, project. Notice how succinct F# is:

let prepare (proj:Project) =
  let app = new ApplicationClass()
  app.Visible <- true
  let p = app.Projects.Add()
  p.Name <- proj.Name
  proj.Resources |> List.iter(fun r ->
    let r' = p.Resources.Add()
    r'.Name <- r.Position // position, not name :)
    let tables = r'.CostRateTables
    let table = tables.[1]
    table.PayRates.[1].StandardRate <- r.Rate
    table.PayRates.[1].OvertimeRate <- (r.Rate + (r.Rate >>> 1)))
  // make root task with project name
  let root = p.Tasks.Add()
  root.Name <- proj.Name
  // add groups
  proj.Groups |> List.rev |> List.iter(fun g -> 
    let t = p.Tasks.Add()
    t.Name <- g.Name
    t.OutlineLevel <- 2s
    // who is responsible for this group?
    t.ResourceNames <- g.Person.Position
    // add tasks
    let tasksInOrder = g.Tasks |> List.rev
    tasksInOrder |> List.iter(fun t' ->
        let t'' = p.Tasks.Add(t'.Name)
        t''.Duration <- t'.Duration
        t''.OutlineLevel <- 3s
        // make task follow previous
        let idx = tasksInOrder |> List.findIndex(fun f -> f.Equals(t'))
        if (idx > 0) then 
          t''.Predecessors <- Convert.ToString(t''.Index - 1)

Yep, we finally reverse those backward lists with List.rev – probably not the fastest operation in the world, but I don't care. All that matters is that the script runs and gives us the results we want – resource definitions, group names and tasks which are grouped and linked. What more could a PM ask for? (Quite a bit actually, but that’s another story.)

A complete project definition can therefore look like this:

project "F# DSL Article" starts "01/01/2009"
resource "Dmitri" isa "Writer" with_rate 140
resource "Computer" isa "Dumb Machine" with_rate 0
group "DSL Popularization" done_by "Dmitri"
task "Create basic estimation DSL" takes 1 day
task "Write article" takes 1 day
task "Post article and wait for comments" takes 1 week
group "Infrastructure Support" done_by "Computer"
task "Provide VS2010 and MS Project" takes 1 day
task "Download and deploy TypograFix" takes 1 day
task "Sit idly while owner waits for comments" takes 1 week
prepare my_project


This article shows that making a DSL in F# is really simple. Of course, the thing about DSLs is they are domain-specific, so for the domain you choose you might encounter a lot more challenges. Have fun!


  • 16th August, 2009: Initial post


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Written By
Founder ActiveMesa
United Kingdom United Kingdom
I work primarily with the .NET technology stack, and specialize in accelerated code production via code generation (static or dynamic), aspect-oriented programming, MDA, domain-specific languages and anything else that gets products out the door faster. My languages of choice are C# and C++, though I'm open to suggestions.

Comments and Discussions

QuestionAwsam Pin
rajnishsharma3-May-13 0:07
professionalrajnishsharma3-May-13 0:07 
GeneralMy vote of 5 Pin
Hamilton Jose19-Apr-13 14:23
Hamilton Jose19-Apr-13 14:23 
GeneralLanguage Pin
Member 31259029-Aug-09 10:10
Member 31259029-Aug-09 10:10 
GeneralRe: Language Pin
Dmitri Nеstеruk1-Apr-13 3:06
Dmitri Nеstеruk1-Apr-13 3:06 
GeneralExcellent article Pin
Josh Fischer25-Aug-09 6:14
Josh Fischer25-Aug-09 6:14 
GeneralRe: Excellent article Pin
Dmitri Nеstеruk1-Apr-13 3:08
Dmitri Nеstеruk1-Apr-13 3:08 
GeneralNeatly written Pin
Rajesh R Subramanian22-Aug-09 9:26
professionalRajesh R Subramanian22-Aug-09 9:26 
GeneralRe: Neatly written Pin
Dmitri Nеstеruk1-Apr-13 3:08
Dmitri Nеstеruk1-Apr-13 3:08 
GeneralThis really doesn't deserve the 3 votes Pin
Pete O'Hanlon18-Aug-09 10:12
mvePete O'Hanlon18-Aug-09 10:12 
GeneralRe: This really doesn't deserve the 3 votes Pin
Dmitri Nеstеruk18-Aug-09 10:23
Dmitri Nеstеruk18-Aug-09 10:23 
GeneralI liked it. Pin
Rajesh Pillai17-Aug-09 7:11
Rajesh Pillai17-Aug-09 7:11 
GeneralRe: I liked it. Pin
Dmitri Nеstеruk1-Apr-13 3:09
Dmitri Nеstеruk1-Apr-13 3:09 
GeneralExcellent Pin
Rama Krishna Vavilala17-Aug-09 4:18
Rama Krishna Vavilala17-Aug-09 4:18 
GeneralRe: Excellent Pin
Dmitri Nеstеruk17-Aug-09 4:28
Dmitri Nеstеruk17-Aug-09 4:28 

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