Improve Error Reporting: Make FS0072 less intimidating. · Issue #13013 · dotnet/...

The following code leads to the following error:

let bar foo =
    let age = foo.Bar + 99
    age * 2

Lookup on object of indeterminate type based on information prior to this program point. A type annotation may be needed prior to this program point to constrain the type of the object. This may allow the lookup to be resolved.

The message is overly verbose and complicated, using words like "indeterminate" and so on. In reality, all we're trying to say is: "I couldn't infer what this type is! Please add an annotation to tell me directly.". We could also provide some guidance to explain that the compiler cannot generally infer types from C#.

How about:

Unable to infer the type of foo. Please add a type annotation where foo is declared e.g. (foo:string).

It is unclear what type foo is. Please add a type annotation where foo is declared e.g. (foo:string).

We could also add some "background" information as well:

Unable to infer the type of foo. Please add a type annotation where foo is declared e.g. (foo:string). F# can only directly infer from member usage types that are defined in F#; if the type of foo is defined in a C# library (including anything from the Base Class Library e.g. string), F# will not be able to infer it based on a member access.