Implementing a Rule Based System for First Aid Prescriptions

A rule based system for the calculation of medication to be used in an APLS (Acute Pediatric Life Support) setting follows can be nicely implemented using a functional approach.

F# is a relatively new .net family member is the .net version of a functional programming language. Functional programming languages are particularly suitable to solve data processing and decision support problems. So, I decided to give it a go.

First of all, the problem. As medication in children has to applied to a broad range of age and weight categories, different medication solutions have to be applied. For example, for midazolam we use the following solutions:

Weight Category	Solution
< 5.0 kg	50 mg/ 50 mL
5.0 - 15 kg	100 mg/ 50 mL
15 - 23 kg	150 mg/ 50 mL
> 23 kg	200 mg/ 50 mL

In 'normal' C# this would require a lot of if then constructs. In F# the initial solution already looks nicer:

[<Measure>] type kg
[<Measure>] type mg

let selectMidazolamQuantity (weight: float<kg>) :float<mg> =
    match weight with
    | w when w < 0.0<kg> || w > 200.0<kg> -> failwith "Not a valid weight"
    | w when w < 5.0<kg> -> 50.0<mg>
    | w when w >= 5.0<kg> && w < 15.0<kg> -> 100.0<mg>
    | w when w >= 15.0<kg> && w < 23.0<kg> -> 150.0<mg>
    | w when w >= 23.0<kg> -> 200.0<mg>

let qty = selectMidazolamQuantity(23.0<kg>)
qty

In F# instead of if then constructs, pattern matching can be used. The correctness of the pattern is checked at compiler time. So, for example when the pattern is incomplete, this is immediately shown in the editor. Also, primitive values can be decorated with a measure attribute.

When the above code is send to F# interactive this will have the following result:

Script.fsx(40,11): warning FS0025: Incomplete pattern matches on this expression.
val selectMidazolamQuantity : float<kg> -> float<mg>
val qty : float<mg> = 200.0

This code can be further improved by matching against a list of categories:

let milrinoneRules = [| 
    (0.4<kg>, 5.0<kg>, 5.0<mg>); 
    (5.0<kg>, 10.0<kg>, 10.0<mg>);
    (10.0<kg>, 200.0<kg>, 15.0<mg>) 
    |]

let selectQuantity (weight: float<kg>, rules: (float<kg>*float<kg>*float<mg>)[]) : float<mg> =
    match weight with
    | w when w < 0.0<kg> || w > 200.0<kg> -> failwith "Not a valid weight"
    | w ->  
        let rule = Array.find(fun (lower, upper, _) -> w > lower && w <= upper) rules
        let _, _, quantity = rule
        printf "quantity is %s\n" (quantity.ToString())
        quantity

selectQuantity (12.0<kg>, milrinoneRules)

Next step is to annotate the rules set with the medication for wich it is used.