Unhelpful error messages involving Dictionary

bgrommes

Summary

Attempting the following (where _upperToLowerDictionary is a Shared private property used as a backing field)

_upperToLowerDictionary = Dictionary(Of Integer, Integer)

... produces the error, Cannot assign Dictionary to variable of type 'Dictionary'

Looking at the docs, it appears the constructor really demands at least an empty list, so, changed it to:

_upperToLowerDictionary = Dictionary(Of Integer, Integer) = {}

Now the error message is, Expected end of line. Although when I first was drafting it, it was actually Expect '(' after generic type arguments. I think I might have had New between = and Dictionary at the time, or vice versa ... I'm starting to lose track, lol.

So this ended up working:

var dict As Dictionary(Of Integer, Integer) = {}
_upperToLowerDictionary = dict

That seems clumsy / unnecessary and technically it's an extra assignment overhead, though admittedly tiny (not an extra allocation).

I have two very similar methods with the exact same layout and very similar code (just reversing the key, value order into a different dictionary) and the error message, instead of being on the empty line following the attempted instantiation / assignment, was two lines further down, in the middle of a While/Wend loop. In both cases the error diamond should have been on the line itself.

In general I'm seeing a fair bit of squirrelly / fussy behavior, often with off-point error messages like this, but hopefully this example is instructive / helpful. In fairness some of this is my unfamiliarity with the syntax and my acclimatization to C# rather than Xojo, but still.

Objo Studio Version

26.6.5

PaulS

You can add the (Of Integer, Integer) to the Property definition.

After that you can simply say in your source code:

_upperToLowerDictionary = {}

bgrommes

PaulS Already did that:

That said, your abbreviated syntax does work, so thanks for pointing that out!

My original post still stands though; the error messages are still unhelpful and honestly I don't see why any of those other syntaxes don't work.

Garry

Thanks, this was a useful report. There were two separate things going on here.

First, this syntax:

_upperToLowerDictionary = Dictionary(Of Integer, Integer)

is not a dictionary construction expression. Dictionary(Of Integer, Integer) is a type name, not a value. It is valid in places where the grammar expects a type, for example:

Var d As Dictionary(Of Integer, Integer)

or, from the next build ( 😉 ):

Var d As New Dictionary(Of Integer, Integer)

Since your property is already declared as Dictionary(Of Integer, Integer), the shortest correct assignment is:

_upperToLowerDictionary = {}

The empty dictionary literal gets its key/value types from the typed assignment target.

This syntax is also not valid Objo:

_upperToLowerDictionary = Dictionary(Of Integer, Integer) = {}

That is effectively trying to put a type annotation into the right-hand side of an assignment. Objo does not have inline typed dictionary literals in that form.

That said, the compiler diagnostics were definitely poor. In particular:

Cannot assign Dictionary to variable of type 'Dictionary' was hiding the generic arguments, making the error look contradictory.
Expected '(' after generic type arguments was too parser-internal for this case.
Error recovery could put the marker later than the actual bad assignment line.

I’ve fixed these for the next build. The compiler now recognises this specific mistake and reports that Dictionary(Of K, V) is a type, not a value, with a suggestion to use {} in a typed context or New Dictionary(Of K, V). Assignment mismatch errors also now preserve the full parameterised type names.

I’ve also added support for:

Var d As New Dictionary(Of Integer, String)

and:

Var d As Dictionary(Of Integer, String) = New Dictionary(Of Integer, String)

Both create an empty dictionary. Constructor arguments are not supported for Dictionary at the moment; use Reserve(capacity) if you want to reserve storage.

So the original workaround with a temporary variable will no longer be necessary.

Tracked: https://feedback.objo.dev/bug/553