テストをコンピュテーション式で書けないか、という思考実験

TestBuilder.fs

// コンピュテーション式中で用いる型
type AssertionResult<'T> =
    | NotAsserted of 'T
    | Success
    | Failure of string

// コンピュテーションビルダー
type TestBuilder() =
    member self.Bind(x, f) =
        match x with
        | NotAsserted(y) -> f y
        | z              -> z
    member self.Return(_) = Success
    member self.ReturnFrom(x) = x

// アサート用関数
let assertEqual<'T when 'T : equality> (x: 'T) (y: 'T) : AssertionResult<'T> =
    if x = y then
        Success
    else
        Failure(System.String.Format("Expect: {0}\nActual: {1}", x, y))

// テストに名前をつけるためのトリック
let test = fun description -> TestBuilder()

// テスト式
test "We can write good information" {
    let! x = NotAsserted(1)
    let! y = NotAsserted(2)
    return! (assertEqual x y)
}
//=> val it : AssertionResult<int> = Failure "Expect: 1
//   Actual: 2"

輪が広がりを見せている…！

https://gist.github.com/bleis-tift/a7a78d8122b99c083f1a
https://gist.github.com/pocketberserker/40006f8da7e195924713

更なる広がり

https://gist.github.com/kos59125/238d7d67984f87ce106a