feat(gnolang): print nil slices as undefined

[notJoon]

The println function was encountering issues when trying to process nil values(e.g. nil slice). This was causing memory errors. The issue has been addressed by adding checks for nil values and handling them appropriately to prevent memory errors.

related with: #1377

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (1126d9f) 55.98% compared to head (5ee22c9) 56.54%.
Report is 14 commits behind head on master.

Files	Patch %	Lines
gnovm/pkg/gnolang/values_string.go	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1380      +/-   ##
==========================================
+ Coverage   55.98%   56.54%   +0.55%     
==========================================
  Files         420      423       +3     
  Lines       65443    66448    +1005     
==========================================
+ Hits        36640    37570     +930     
- Misses      25940    25986      +46     
- Partials     2863     2892      +29     

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[deelawn]

Awesome. Thanks for addressing my comments. I'll take another look shortly.

[deelawn]

I've been looking at this closely and I think the most correct way to fix this issue may be by following example of how nil function variables are printed -- nil <function type>. So rather than do any kind of check before the type assertion switch case block, the changes could be limited to the complex types here:

gno/gnovm/pkg/gnolang/values_string.go

Lines 246 to 286 in 903ae18

	case *ArrayType:
	return tv.V.(*ArrayValue).String()
	case *SliceType:
	return tv.V.(*SliceValue).String()
	case *StructType:
	return tv.V.(*StructValue).String()
	case *MapType:
	return tv.V.(*MapValue).String()
	case *FuncType:
	switch fv := tv.V.(type) {
	case nil:
	ft := tv.T.String()
	return "nil " + ft
	case *FuncValue:
	return fv.String()
	case *BoundMethodValue:
	return fv.String()
	default:
	panic(fmt.Sprintf(
	"unexpected func type %v",
	reflect.TypeOf(tv.V)))
	}
	case *InterfaceType:
	if debug {
	if tv.DebugHasValue() {
	panic("should not happen")
	}
	}
	return nilStr
	case *TypeType:
	return tv.V.(TypeValue).String()
	case *DeclaredType:
	panic("should not happen")
	case *PackageType:
	return tv.V.(*PackageValue).String()
	case *ChanType:
	panic("not yet implemented")
	// return tv.V.(*ChanValue).String()
	case *NativeType:
	return fmt.Sprintf("%v",
	tv.V.(*NativeValue).Value.Interface())

I think PointerType and InterfaceType can be left as is, along with any other types that just panic. But the other cases that do type assertions on tv.V should add a check to see if it is nil. If it is, then it should result in a string with the value nil <tv.T.String()>.

What do you think about that approach?

[notJoon]

should result in a string with the value nil <tv.T.String()>.

Yes, that seems reasonable. This approach will ensure nil values are treated consistently across all types.

[deelawn]

Nice job; looks good to me 👍

[moul]

LGTM, thank you 👍