Getting rid of the “pyramid of doom” in F#

Question

I have several verbal expressions that I\'ve packaged into one function:

open FsVerbalExpressions
open FsVerbalExpressions.VerbalExpression
open System.Text.         


        

Answer 1

If you are only matching against booleans, then if ... elif is sufficient:

let newInst (x: string) =
    if isMatch x k12VerbEx then
        "K - 12"
    elif isMatch x twoYearCollegeVerbEx then
        "2 Year College"
    elif isMatch x universityVerbEx then
        "University"
    elif isMatch x privateSchoolVerbEx then
        "Private / Charter School"
    else
        "Other"

A more flexible possibility would be to create an active pattern:

let (|IsMatch|_|) f x =
    if isMatch x f then Some () else None

let newInst (x: string) =
    match x with
    | IsMatch k12VerbEx -> "K - 12"
    | IsMatch twoYearCollegeVerbEx -> "2 Year College"
    | IsMatch universityVerbEx -> "University"
    | IsMatch privateSchoolVerbEx -> "Private / Charter School"
    | _ -> "Other"

Answer 2

When there is sequential repetition of exactly the same form of code, I prefer to use a data-driven approach instead:

let verbExStrings =
    [
        (k12VerbEx, "K - 12")
        (twoYearCollegeVerbEx, "2 Year College")
        (universityVerbEx, "University")
        (privateSchoolVerbEx, "Private / Charter School")
    ]

let newInst x =
    verbExStrings
    |> List.tryPick (fun (verbEx, string) -> if isMatch x verbEx then Some string else None)
    |> function Some x -> x | _ -> "Other"

An advantage of this approach is that the raw data (verbExStrings) can come in handy in other places and is not tied to your code implementation.