Question

OK, i’m used to good old C assertions, when they fail they output the condition they failed on, and the line it was on. (implimented with the preprocessor).

Now I’m programming in F# (a .net language), got code full of assert. (trying to program defensivly). and my assets show a popup with the call stack. according to the documentation could also pass the assert a string to display when it fails. so i could make it pass a string copy of my condition. eg:

assert (x=true, "x=true")

but this violate the Duplication (avoiding) principal.

If I later decide to change what I want to assert. (maybe i realised i was asserting something to be false when i should have been asserting it to be true) then I have to change the code in two places, otherwise the message won’t match what I’m actually checking. and if i forget things could be really confusing.

How is this normally dealt with?

Answer 1

Echoing what Tomas said, you can use quotations to avoid duplication. But instead of ToLinqExpression -> ToString, which apparently produces C# source code, you can use my library Unquote which produces F# source code strings. Combine this with an implementation suitable for production code (i.e. defining Assert inline with a non-DEBUG compilation symbol branch emitting only the no-op () means uses of Assert will be completely removed in non-DEBUG builds with optimizations turned on):

let inline Assert (q:Expr<bool>) =
    #if DEBUG
        System.Diagnostics.Debug.Assert(q.Eval(), q.Decompile())
    #else
        ()
    #endif

then the assertion expression Assert <@ (22 + 2) / 2 = ("assert" |> String.length) @> produces the following popup dialog by default (I think you can configure System.Diagnostics.Debug behavior):

Where in comparison q.ToLinqExpression().ToString() produces (((22 + 2) / 2) == op_PipeRight("assert", ToFSharpFunc(str => Length(str)))).

In fact, Unquote was designed specifically for the purpose of producing helpful unit test assertion failure messages, and can easily be adapted to debug assertions producing step-by-step failure messages:

let inline Assert (q:Expr<bool>) =
    #if DEBUG
        System.Diagnostics.Debug.Assert(q.Eval(), q.ReduceFully() |> List.map (fun q -> q.Decompile()) |> String.concat System.Environment.NewLine)
    #else
        ()
    #endif