I'm having a problem with creating an array of objects of a class type I created in dafny. The problem is when initialising a new array of that type I'm getting this error in vscode:
unless an initializer is provided for the array elements, a new array of 'Cup' must have empty size
This is the code (actually a stripped back version that still illustrates the problem):
datatype Drink = WATER | LEMONADE | COFFEE | TEA
class Cup {
var volume: int
var drink_type: Drink
var dirty: bool
predicate Valid()
reads this;
{
volume >= 0
}
constructor (v: int, dt: Drink, d: bool)
requires v >= 0;
ensures Valid();
{
volume := v;
drink_type := dt;
dirty := d;
}
}
method FilterCupDrinkType(a: array<Cup>, dt: Drink) returns (b: array<Cup>, n: int)
{
var temp := new Cup[a.Length];
}
I looked through the manual and online but couldn't really find an answer so I'm hoping someone here knows what to do. If its not possible to do this in dafny(very new to dafny) I would appreciate any suggestions on verifying something like this. Thanks!
You could create a default Cup and then initialize the array with it as follows
method FilterCupDrinkType(a: array<Cup>, dt: Drink) returns (b: array<Cup>, n: int)
{
var default := new Cup(0, WATER, false);
var temp := new Cup[a.Length](_ => default);
}
or you could allow temp to be an array of nullable Cups (i.e., Cup?)
method FilterCupDrinkType(a: array<Cup>, dt: Drink) returns (b: array<Cup>, n: int)
{
var temp := new Cup?[a.Length];
}
or you can copy a as follows
method FilterCupDrinkType(a: array<Cup>, dt: Drink) returns (b: array<Cup>, n: int)
{
var temp := new Cup[a.Length](i requires 0 <= i < a.Length reads a => a[i]);
}
Often a good way to find solutions for such questions, if you don't want to wait for an answer here, is to search through Dafny's extensive test suite at https://github.com/dafny-lang/dafny/tree/master/Test. Of course the tutorial is a better choice if treats the topic.
Related
In service A I have a string that get hashed like this:
fun String.toHash(): Long {
var hashCode = this.hashCode().toLong()
if (hashCode < 0L) {
hashCode *= -1
}
return hashCode
}
I want to replicate this code in service B written in Golang so for the same word I get the exact same hash. For what I understand from Kotlin's documentation the hash applied returns a 64bit integer. So in Go I am doing this:
func hash(s string) int64 {
h := fnv.New64()
h.Write([]byte(s))
v := h.Sum64()
return int64(v)
}
But while unit testing this I do not get the same value. I get:
func Test_hash(t *testing.T) {
tests := []struct {
input string
output int64
}{
{input: "papafritas", output: 1079370635},
}
for _, test := range tests {
got := hash(test.input)
assert.Equal(t, test.output, got)
}
}
Result:
7841672725449611742
Am I doing something wrong?
Java and therefore Kotlin uses different hash function than Go.
Possible options are:
Use a standard hash function.
Reimplement Java hashCode for Strings in Go.
I'm trying to build a simple orm layer for golang.
Which would take a struct and generate the cols [] which can then be passed to sql function
rows.Scan(cols...) which takes pointers of fields in the struct corresponding to each of the columns it has found in the result set
Here is my example struct
type ExampleStruct struct {
ID int64 `sql:"id"`
aID string `sql:"a_id"`
UserID int64 `sql:"user_id"`
And this is my generic ORM function
func GetSqlColumnToFieldMap(model *ExampleStruct) map[string]interface{} {
typeOfModel := reflect.TypeOf(*model)
ValueOfModel := reflect.ValueOf(*model)
columnToDataPointerMap := make(map[string]interface{})
for i := 0; i < ValueOfModel.NumField(); i++ {
sql_column := typeOfModel.Field(i).Tag.Get("sql")
structValue := ValueOfModel.Field(i)
columnToDataPointerMap[sql_column] = structValue.Addr()
}
return columnToDataPointerMap
}
Once this method works fine i can use the map it generates to create an ordered list of sql pointers according to the column_names i get in rows() object
However i get below error on the .Addr() method call
panic: reflect.Value.Addr of unaddressable value [recovered]
panic: reflect.Value.Addr of unaddressable value
Is it not possible to do this ?
Also in an ideal scenario i would want the method to take an interface instead of *ExampleStruct so that it can be reused across different db models.
The error says the value whose address you want to get is unaddressable. This is because even though you pass a pointer to GetSqlColumnToFieldMap(), you immediately dereference it and work with a non-pointer value later on.
This value is wrapped in an interface{} when passed to reflect.ValueOf(), and values wrappped in interfaces are not addressable.
You must not dereference the pointer, but instead use Type.Elem() and Value.Elem() to get the element type and pointed value.
Something like this:
func GetSqlColumnToFieldMap(model *ExampleStruct) map[string]interface{} {
t := reflect.TypeOf(model).Elem()
v := reflect.ValueOf(model).Elem()
columnToDataPointerMap := make(map[string]interface{})
for i := 0; i < v.NumField(); i++ {
sql_column := t.Field(i).Tag.Get("sql")
structValue := v.Field(i)
columnToDataPointerMap[sql_column] = structValue.Addr()
}
return columnToDataPointerMap
}
With this simple change it works! And it doesn't depend on the parameter type, you may change it to interface{} and pass any struct pointers.
func GetSqlColumnToFieldMap(model interface{}) map[string]interface{} {
// ...
}
Testing it:
type ExampleStruct struct {
ID int64 `sql:"id"`
AID string `sql:"a_id"`
UserID int64 `sql:"user_id"`
}
type Point struct {
X int `sql:"x"`
Y int `sql:"y"`
}
func main() {
fmt.Println(GetSqlColumnToFieldMap(&ExampleStruct{}))
fmt.Println(GetSqlColumnToFieldMap(&Point{}))
}
Output (try it on the Go Playground):
map[a_id:<*string Value> id:<*int64 Value> user_id:<*int64 Value>]
map[x:<*int Value> y:<*int Value>]
Note that Value.Addr() returns the address wrapped in a reflect.Value. To "unwrap" the pointer, use Value.Interface():
func GetSqlColumnToFieldMap(model interface{}) map[string]interface{} {
t := reflect.TypeOf(model).Elem()
v := reflect.ValueOf(model).Elem()
m := make(map[string]interface{})
for i := 0; i < v.NumField(); i++ {
colName := t.Field(i).Tag.Get("sql")
field := v.Field(i)
m[colName] = field.Addr().Interface()
}
return m
}
This will output (try it on the Go Playground):
map[a_id:0xc00007e008 id:0xc00007e000 user_id:0xc00007e018]
map[x:0xc000018060 y:0xc000018068]
For an in-depth introduction to reflection, please read blog post: The Laws of Reflection
I am having problems getting the golang validator to work with SQL null types. Here's an example of what I tried:
package main
import (
"database/sql"
"database/sql/driver"
"log"
"gopkg.in/go-playground/validator.v9"
)
// NullInt64
type NullInt64 struct {
sql.NullInt64
Set bool
}
func MakeNullInt64(valid bool, val int64) NullInt64 {
n := NullInt64{}
n.Set = true
n.Valid = valid
if valid {
n.Int64 = val
}
return n
}
func (n *NullInt64) Value() (driver.Value, error) {
if !n.NullInt64.Valid {
return nil, nil
}
return n.NullInt64.Int64, nil
}
type Thing struct {
N2 NullInt64 `validate:"min=10"`
N3 int64 `validate:"min=10"`
N4 *int64 `validate:"min=10"`
}
func main() {
validate := validator.New()
n := int64(6)
number := MakeNullInt64(true, n)
thing := Thing{number, n, &n}
e := validate.Struct(thing)
log.Println(e)
}
When I run this code, I only get this output:
Key: 'Thing.N3' Error:Field validation for 'N3' failed on the 'min'
tag
Key: 'Thing.N4' Error:Field validation for 'N4' failed on the
'min' tag
The problem is that I want it to also show that Thing.N2 failed for the same reasons as Thing.N3 and Thing.N4.
I tried introducing the func (n *NullInt64) Value() method because it was mentioned in the documentation. But I think I misunderstood something. Can anyone tell me what I did wrong?
UPDATE
There is an Example specifically for that. You may check it out. My other proposed solution should still work though.
Since the value you are trying to validate is Int64 inside sql.NullInt64, the easiest way would be to remove the validate tag and just register a Struct Level validation using:
validate.RegisterStructValidation(NullInt64StructLevelValidation, NullInt64{})
while NullInt64StructLevelValidation is a StructLevelFunc that looks like this:
func NullInt64StructLevelValidation(sl validator.StructLevel) {
ni := sl.Current().Interface().(NullInt64)
if ni.NullInt64.Int64 < 10 {
sl.ReportError(ni.NullInt64.Int64, "Int64", "", "min", "")
}
}
Note #1: this line thing := Thing{number,&number,n,&n} has one argument too many. I assume you meant thing := Thing{number, n, &n}
Note #2: Go tooling including gofmt is considered to be one of the most powerful features of the language. Please consider using it/them.
EDIT #1:
I don't think implementing Valuer interface is of any value in this context.
I have a tree of structs which I'd like to test using testing/quick, but constraining it to within my invariants.
This example code works:
var rnd = rand.New(rand.NewSource(time.Now().UnixNano()))
type X struct {
HasChildren bool
Children []*X
}
func TestSomething(t *testing.T) {
x, _ := quick.Value(reflect.TypeOf(X{}), rnd)
_ = x
// test some stuff here
}
But we hold HasChildren = true whenever len(Children) > 0 as an invariant, so it'd be better to ensure that whatever quick.Value() generates respects that (rather than finding "bugs" that don't actually exist).
I figured I could define a Generate function which uses quick.Value() to populate all the variable members:
func (X) Generate(rand *rand.Rand, size int) reflect.Value {
x := X{}
throwaway, _ := quick.Value(reflect.TypeOf([]*X{}), rand)
x.Children = throwaway.Interface().([]*X)
if len(x.Children) > 0 {
x.HasChildren = true
} else {
x.HasChildren = false
}
return reflect.ValueOf(x)
}
But this is panicking:
panic: value method main.X.Generate called using nil *X pointer [recovered]
And when I change Children from []*X to []X, it dies with a stack overflow.
The documentation is very thin on examples, and I'm finding almost nothing in web searches either.
How can this be done?
Looking at the testing/quick source code it seems that you can't create recursive custom generators and at the same time reuse the quick library facilities to generate the array part of the struct, because the size parameter, that is designed to limit the number of recursive calls, cannot be passed back into quick.Value(...)
https://golang.org/src/testing/quick/quick.go (see around line 50)
in your case this lead to an infinite tree that quickly "explodes" with 1..50 leafs at each level (that's the reason for the stack overflow).
If the function quick.sizedValue() had been public we could have used it to accomplish your task, but unfortunately this is not the case.
BTW since HasChildren is an invariant, can't you simply make it a struct method?
type X struct {
Children []*X
}
func (me *X) HasChildren() bool {
return len(me.Children) > 0
}
func main() {
.... generate X ....
if x.HasChildren() {
.....
}
}
I was wondering if it is possible to dynamically create variables in Go?
I have provided a pseudo-code below to illustrate what I mean. I am storing the newly created variables in a slice:
func method() {
slice := make([]type)
for(i=0;i<10;i++)
{
var variable+i=i;
slice := append(slice, variablei)
}
}
At the end of the loop, the slice should contain the variables: variable1, variable2...variable9
Go has no dynamic variables.
Dynamic variables in most languages are implemented as Map (Hashtable).
So you can have one of following maps in your code that will do what you want
var m1 map[string]int
var m2 map[string]string
var m3 map[string]interface{}
here is Go code that does what you what
http://play.golang.org/p/d4aKTi1OB0
package main
import "fmt"
func method() []int {
var slice []int
for i := 0; i < 10; i++ {
m1 := map[string]int{}
key := fmt.Sprintf("variable%d", i)
m1[key] = i
slice = append(slice, m1[key])
}
return slice
}
func main() {
fmt.Println(method())
}
No; you cannot refer to local variables if you don’t know their names at compile-time.
If you need the extra indirection you can do it using pointers instead.
func function() {
slice := []*int{}
for i := 0; i < 10; i++ {
variable := i
slice = append(slice, &variable)
}
// slice now contains ten pointers to integers
}
Also note that the parentheses in the for loop ought to be omitted, and putting the opening brace on a new line is a syntax error due to automatic semicolon insertion after ++. makeing a slice requires you to pass a length, hence I don’t use it since append is used anyway.