Weird inconsistency using Assert

Question

I'm writing some unit tests in F# and have noticed a very strange inconsistency in how the Assert functions work. See below. Why doesn't the first example compile?

        let x = 4
        let y = 4

        // Won't compile; error is "The member or object constructor
        // 'IsTrue' does not take 0 argument(s)."
        Assert.IsTrue(x=y)

        // Works just fine.
        Assert.IsTrue(4=4)
        Assert.IsFalse(4=5)

        // Work as expected, but why the extra parenthesis? What do
        // they signify?
        Assert.IsTrue((x=y)) 

        // Works as expected, but how is this different than the 
        // first Assert above?
        Assert.IsTrue(x.GetHashCode()=y.GetHashCode())

Answer 1

What library/assembly your Assert object from?

What F# shows as signature for Assert.IsTrue() function?

---

Petr

Answer 2

I'm using the Assert function from Microsoft.VisualStudio.TestTools.UnitTesting. The signature shows Assert.IsTrue(condition: bool) : unit

Answer 3

Well it looks like bug to me. For example this function will work:

let myassert (exp: bool) = if exp = false then failwith "asert" else () 
myassert(x=y)

---

Petr

Answer 4

Microsoft.VisualStudio.TestTools.UnitTesting.Assert.IsTrue(x=y) //error FS0505
System.Diagnostics.Debug.Assert(x=y) //error FS0505

but when i write:

let test = Microsoft.VisualStudio.TestTools.UnitTesting.Assert.IsTrue
test(x=y)

and:

assert(x=y) //'assert' is a keyword

it works. maybe it's about the syntax analysis, and i found this:

https://github.com/fsharp/fsharp/blob/master/src/fsharp/tc.fs //line: 6263

//-------------------------------------------------------------------------

// TcAssertExpr

//-------------------------------------------------------------------------

// Check an 'assert(x)' expression.

and TcAssertExpr cenv overallTy env (m:range) tpenv x  =

    let synm = m.MakeSynthetic() // Mark as synthetic so the language service won't pick it up.

    let callDiagnosticsExpr = SynExpr.App(ExprAtomicFlag.Atomic, false, mkSynLidGet synm ["System";"Diagnostics";"Debug"] "Assert",

                                           // wrap an extra parentheses so 'assert(x=1) isn't considered a named argument to a method call

                                           SynExpr.Paren(x,range0,None,synm), synm)

    TcExpr cenv overallTy env tpenv callDiagnosticsExpr

Answer 5

F# supports named arguments and also something called "settable return properties".  The basic idea is that you can set properties of an object as part of a constructor or method call:

type T() =
    member val IntValue = 4 with get, set
    static member Create() = T()

let t1 = T(IntValue = 1)
let t2 = T.Create(IntValue = 4)

Unfortunately, in rare cases such as yours a Boolean expression can look like trying to set an argument by name or performing one of these property assignments, and the compiler won't be able to parse your call as you expect.  The workaround is to parenthesize it, as you've discovered.