f_sharp — consolidated language guide — DedSec Project

F#

Mental model

F# is an ML-family language on .NET with type inference, immutable values and strong support for discriminated unions.

Minimal source sample

let squares = [1; 2; 3] |> List.map (fun x -> x * x)
printfn "%A" squares

Experiment to carry out

Trace the pipe operator and compare list with array operations before using a collection in a pipeline.

A specific failure to distinguish

The type checker infers concrete collection types; do not assume a C# List<T> is the same as an F# list.

To evaluate the exercise, record the initial input, predicted behavior and actual observation in a compatible F_Sharp environment. Add a second run changing only one variable or argument; this makes the comparison attributable to a concrete cause. If the required runtime is not installed, treat the example as unexecuted source analysis and never label an imagined output as a passing test.

F_Sharp — Expanded practice workbook

Run a script with explicit outcomes

F# runs on .NET but has its own expression-oriented syntax and algebraic data types. Save parse.fsx:

let parseScore (text: string) =
    match System.Int32.TryParse(text) with
    | true, score -> Some score
    | false, _ -> None

assert (parseScore "12" = Some 12)
assert (parseScore "bad" = None)
assert (parseScore "0" = Some 0)
printfn "%A" (parseScore "-3")

Using a suitable .NET SDK, run dotnet fsi parse.fsx. The predicted output is Some -3. The Some and None cases are distinct from legitimate zero; consumers must pattern-match or otherwise handle the absent value. Record .NET SDK and F# compiler versions before comparing results with another machine.

Separate type checking from business validation

The parser accepts -3 as an integer. If a score must be nonnegative, that is a separate rule. Write a function returning Result<int,string> (or equivalent inferred type) with Ok for valid values and Error for malformed or negative input. Explain how the error description travels to the caller without raising a generic exception.

A controlled type mistake

Temporarily change the success result to Some "twelve" while keeping the failure result None, and compare inferred types with the integer-returning original. The compiler may infer an option of strings; the later equality assertion against Some 12 then exposes the contract mismatch. Capture the actual diagnostic instead of guessing at the message wording.

Independent extension

Process a list ["3"; "bad"; "0"; "-1"]. Use List.map and then explicitly choose whether invalid entries are rejected, grouped as errors or dropped. Compare List.choose with List.map: dropping None loses positional information. Add an empty list and very large numeric text. Reference: https://learn.microsoft.com/dotnet/fsharp/

F_Sharp — Practical runnable code lab

How to run the samples

These independent samples use the appropriate language toolchain. Representative command: dotnet fsi {file}. Replace placeholders with the actual filename, keep test data isolated, and do not interpret this as automatic installation.