sum types should have a default 0 value for the first type mentioned in the list of types.

[JalonSolov]

Describe the bug

sum types do not have a default value. This can lead to errors if you try something as simple as

x := MyType{}

this could be easily fixed if the default was to assign the default 0 value for the first type in the list. Users could change which type was assigned by changing the order of types in the sum type.

Note that this change has been approved by Alex: https://discord.com/channels/592103645835821068/592106336838352923/1272226313314242673

Reproduction Steps

type MyType = int | string

x := MyType{}

println(x)

Expected Behavior

0

Current Behavior

x.v:3:6: notice: direct sum type init (`x := SumType{}`) will be removed soon
    1 | type MyType = int | string
    2 | 
    3 | x := MyType{}
      |      ~~~~~~~~
    4 | 
    5 | println(x)
unknown sum type value

Possible Solution

No response

Additional Information/Context

No response

V version

V 0.4.7 20a525a

Environment details (OS name and version, etc.)

V full version: V 0.4.7 efa98d9.20a525a
OS: linux, "Manjaro Linux"
Processor: 32 cpus, 64bit, little endian, AMD Ryzen 9 7950X 16-Core Processor

getwd: /home/jalon
vexe: /home/jalon/git/v/v
vexe mtime: 2024-08-11 16:45:16

vroot: OK, value: /home/jalon/git/v
VMODULES: OK, value: /home/jalon/.vmodules
VTMP: OK, value: /tmp/v_1000

Git version: git version 2.46.0
Git vroot status: weekly.2024.32-24-g20a525ae
.git/config present: true

CC version: cc (GCC) 14.1.1 20240720
thirdparty/tcc status: thirdparty-linux-amd64 40e5cbb5

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[spytheman]

The same is true for interface values too - they also do not have default values.

For them, the problem is mitigated a bit because we do have an error for some situations involving them:

a.v:2:6: error: cannot instantiate interface `Abc`
    1 | interface Abc{}
    2 | a := Abc{}
      |      ~~~~~
    3 | dump(a)
#1 20:03:14 ^ master /v/oo>

but others are still allowed:

a.v:2:6: warning: arrays of interfaces need to be initialized right away, therefore `len:` cannot be used (unless inside `unsafe`, or if you also use `init:`)
    1 | interface Abc{}
    2 | a := []Abc{len:4}
      |      ~~~~~~
    3 | dump(a)
[a.v:3] a: [unknown interface value, unknown interface value, unknown interface value, unknown interface value]

and even runtime errors, with no complains from the compiler:

interface Abc{}
struct MyStruct { i Abc }
a := []MyStruct{len:4}
dump(a)

producing:

#148 20:05:19 ^ master /v/oo>v run a.v
/tmp/v_1000/a.01J516DMX1F1X3GBBFX1R951GE.tmp.c:2277: at indent_main__Abc_str: RUNTIME ERROR: invalid memory access
/tmp/v_1000/a.01J516DMX1F1X3GBBFX1R951GE.tmp.c:2264: by indent_main__MyStruct_str
/tmp/v_1000/a.01J516DMX1F1X3GBBFX1R951GE.tmp.c:2216: by indent_Array_main__MyStruct_str
/tmp/v_1000/a.01J516DMX1F1X3GBBFX1R951GE.tmp.c:2210: by Array_main__MyStruct_str
/tmp/v_1000/a.01J516DMX1F1X3GBBFX1R951GE.tmp.c:2233: by _v_dump_expr_Array_main__MyStruct
/tmp/v_1000/a.01J516DMX1F1X3GBBFX1R951GE.tmp.c:13400: by main__main

[spytheman]

One problem that will have to be solved too, when the first variant type of a sumtype declaration, gets used to define the default value, is that currently v fmt -w . will reorder the sumtype variants in alphabetical order, and it should not, since the first type will have a semantic meaning in the future.

[JalonSolov]

It could always alphabetize all the types after the first, but that seems like more trouble than it's worth.

Changing the defaults for interfaces may also need it's own separate issue... seems like a much bigger problem than sum types.

[medvednikov]

Just wanted to confirm that agree with this notion.