Feature: support named and unnamed type assignment

[piux2]

Context:

Thank you @r3v4s for identifying the issue.

#651

Thank you @ltzmaxwell for providing a solution.
#674

Root cause is the incomplete support of the spec of named type, which is previously noted in the preprocess.go

Here is the example and go specification

https://go.dev/play/p/uWkp8HXZkAo

    package main

    import "fmt"

    type nat []int

    func main() {
        var a nat
        var b []int

        a = []int{0}
        b = nat{1}
        fmt.Printf("a= %v%T\n", a, a)
        fmt.Printf("b= %v%T\n", b, b)

        a = b
        b = a
        fmt.Printf("a= %v%T\n", a, a)
        fmt.Printf("b= %v%T\n", b, b)

    }

// Output

  a= [0]main.nat
  b= [1][]int
  a= [1]main.nat
  b= [1][]int

https://go.dev/ref/spec#Assignability

Assignability

• V and T have identical underlying types but are not type parameters and at least one of V or T is not a named type.
• V and T are channel types with identical element types, V is a bidirectional channel, and at least one of V or T is not a named type.

https://go.dev/ref/spec#Assignment_statements

Assignment statements
An assignment replaces the current value stored in a variable with a new value specified by an expression.

In assignments, each value must be assignable to the type of the operand to which it is assigned, with the following special cases:

• Any typed value may be assigned to the blank identifier.
• If an untyped constant is assigned to a variable of interface type or the blank identifier, the constant is first implicitly converted to its default type.
• If an untyped boolean value is assigned to a variable of interface type or the blank identifier, it is first implicitly converted to type bool.

Solution

To complete the named and un-name type assignment specs, within existing preprocessing flow while keeping future expanding capability, there are many things need to be considered

Here is the basic idea:

Even though this value assignment happens at runtime, we want to handle it during preprocessing by converting unnamed types to named type, and vice versa, when the assignment happens. This will save logic execution steps when a MsgCall transaction is executed during runtime.

1. identify all named and un-named types of gno type and tag them with IsNamed() method as a type property.

   • *DeclaredType and PrimitiveType are all named types.
   • FieldType, TypeType, PackageType, blockType, *tupleType, RefType can all panic as "unexpected".
   • Using go reflection Type.Name() to determine.

2. Identify when name and unnamed assignment happens

   • general assignment: a,b = c,d
   • assign values to return results func () (uType){ some values assign to return results }
   • a = x()
   • copy (a, b)
   • append(a, b)

3. there is a special case of assigning returned results from a func call to more than one variables

      a,b = x()
   

   This occurs at runtime, but we want to handle the conversion in the preprocessing stage by decomposing it into one 'define' statement, which does not involve conversion, and one assignment statement, which involves conversion as in a normal assignment, like 'a,b = c,d'. This is quite complex, and many more details need to be considered. We will explain the implementation later.

4. corner cases to verify

   • func() call return interfaces in stead of type
   • assign to blank identifier "_"

5. convert right hand sides to Const Type Call Expr according to the left hand side type.

6. add print named type in prinln() so that we can verify all the unnamed and named type in file tests.

   type myInt int
   func main(){
       a = myint(1)
       println(a) 
   }
    // Output
    // (1 main.myInt )
   

Implementation:

The full implementation of entire code changes are located here https://github.com/piux2/gno/pull/2/files for review.

file changes:

• gnovm/pkg/gnolang

  preprocess.go
  transcribe.go
  nodes.go
  types.go
  values.go
  values_string.go
  

• contracts:
  this directory is newly created for easy review and regression test of this feature, we will find proper place to merge tests to main branch

  newly added 80 testing cases for this feature.

• tests/files:

  println() added missing named type to the variables to the output of the file tests.

To make it easier for us to review and and reduce the risk of introducing side effect and hard to find bugs, we break the entire code changes to a few PRs. First is to add named type in println() results. This need to be merged first, otherwise we will not be able to merge rest of code with proper testing cases integrated in CI. #1143