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 Go to latest Published: Dec 9, 2017 License: Apache-2.0 Imports: 10 Imported by: 21

Documentation

Overview

Package analysis performs static program data flow analysis of an API file.

Index

Constants

This section is empty.

Variables

This section is empty.

Functions

This section is empty.

Types

type BoolValue 

type BoolValue struct {
	Possibility
}

BoolValue is an implementation of Value that represents all the possible values of a boolean type.

func (*BoolValue) And 

func (v *BoolValue) And(o *BoolValue) *BoolValue

And returns the logical-and of v and o.

func (*BoolValue) Clone 

func (v *BoolValue) Clone() Value

Clone returns a new instance of BoolValue initialized from v.

func (*BoolValue) Difference 

func (v *BoolValue) Difference(o Value) Value

Difference returns the possibile for v that are not found in o. o must be of type BoolValue.

func (*BoolValue) Equals 

func (v *BoolValue) Equals(o Value) Possibility

Equals returns the possibility of v equaling o. o must be of type BoolValue.

func (*BoolValue) Equivalent 

func (v *BoolValue) Equivalent(o Value) bool

Equivalent returns true iff v and o are equivalent. See Value for the definition of equivalency.

func (*BoolValue) Intersect 

func (v *BoolValue) Intersect(o Value) Value

Intersect returns the intersection of possibile values for v and o. o must be of type BoolValue.

func (*BoolValue) Not 

func (v *BoolValue) Not() *BoolValue

Not returns the logical negation of v.

func (*BoolValue) Or 

func (v *BoolValue) Or(o *BoolValue) *BoolValue

Or returns the logical-or of v and o.

func (*BoolValue) Print 

func (v *BoolValue) Print(results *Results) string

Print returns a textual representation of the value.

func (*BoolValue) String 

func (v *BoolValue) String() string

func (*BoolValue) Type 

func (v *BoolValue) Type() semantic.Type

Type returns semantic.BoolType.

func (*BoolValue) Union 

func (v *BoolValue) Union(o Value) Value

Union returns the union of possibile values for v and o. o must be of type BoolValue.

func (*BoolValue) Valid 

func (v *BoolValue) Valid() bool

Valid returns true if there is any possibility of this value equaling any other.

type Callstack 

type Callstack []CallstackEntry

Callstack holds the callstack to a point in the API file.

func (Callstack) At 

func (c Callstack) At() parse.Node

At returns the parse node to the deepest point in the callstack.

func (Callstack) String 

func (c Callstack) String() string

type CallstackEntry 

type CallstackEntry struct {
	Location   parse.Node
	Function   *semantic.Function
	Parameters map[*semantic.Parameter]Value
}

CallstackEntry is a single entry in a callstack.

func (CallstackEntry) String 

func (e CallstackEntry) String() string

type ClassValue 

type ClassValue struct {
	// The class type.
	Class *semantic.Class

	// A map of field name to possible values for that field.
	Fields map[string]Value
}

ClassValue is an implementation of Value that represents all the possible values of a class type.

func (*ClassValue) Clone 

func (v *ClassValue) Clone() Value

Clone returns a copy of v with a unique pointer.

func (*ClassValue) Difference 

func (v *ClassValue) Difference(o Value) Value

Difference (\) returns the class value with field values that are found in v but not found in o. o must be of type *ClassValue.

func (*ClassValue) Equals 

func (v *ClassValue) Equals(o Value) Possibility

Equals returns the possibility of all fields of v being equal to those in o. o must be of type *ClassValue.

func (*ClassValue) Equivalent 

func (v *ClassValue) Equivalent(o Value) bool

Equivalent returns true iff v and o are equivalent. Unlike Equals() which returns the possibility of two values being equal, Equivalent() returns true iff the set of possible field values are exactly equal. o must be of type *ClassValue.

func (*ClassValue) Intersect 

func (v *ClassValue) Intersect(o Value) Value

Intersect (∩) returns the class value with field values that are found in v and o. o must be of type *ClassValue.

func (*ClassValue) Print 

func (v *ClassValue) Print(results *Results) string

Print returns a textual representation of the value.

func (*ClassValue) Type 

func (v *ClassValue) Type() semantic.Type

Type returns the semantic class type of the value.

func (*ClassValue) Union 

func (v *ClassValue) Union(o Value) Value

Union (∪) returns the class value with field values that are found in v or o. o must be of type *ClassValue.

func (*ClassValue) Valid 

func (v *ClassValue) Valid() bool

Valid returns true if there is any possibility of this value equaling any other.

type EnumValue 

type EnumValue struct {
	Ty      *semantic.Enum
	Numbers *UintValue
	Labels  Labels
}

EnumValue is an implementation of Value that represents all the possible values of an enumerator.

func (*EnumValue) Clone 

func (v *EnumValue) Clone() Value

Clone returns a copy of v with a unique pointer.

func (*EnumValue) Difference 

func (v *EnumValue) Difference(o Value) Value

Difference (\) returns the values that are found in v but not found in o. o must be of type *EnumValue.

func (*EnumValue) Equals 

func (v *EnumValue) Equals(o Value) Possibility

Equals returns the possibility of v being equal to o. o must be of type *EnumValue.

func (*EnumValue) Equivalent 

func (v *EnumValue) Equivalent(o Value) bool

Equivalent returns true iff v and o are equivalent. Unlike Equals() which returns the possibility of two values being equal, Equivalent() returns true iff the set of possible values are exactly equal. o must be of type *EnumValue.

func (*EnumValue) GreaterEqual 

func (v *EnumValue) GreaterEqual(o Value) Possibility

GreaterEqual returns the possibility of v being greater or equal to o. o must be of type *EnumValue.

func (*EnumValue) GreaterThan 

func (v *EnumValue) GreaterThan(o Value) Possibility

GreaterThan returns the possibility of v being greater than o. o must be of type *EnumValue.

func (*EnumValue) Intersect 

func (v *EnumValue) Intersect(o Value) Value

Intersect (∩) returns the values that are found in both v and o. o must be of type *EnumValue.

func (*EnumValue) LessEqual 

func (v *EnumValue) LessEqual(o Value) Possibility

LessEqual returns the possibility of v being less than or equal to o. o must be of type *EnumValue.

func (*EnumValue) LessThan 

func (v *EnumValue) LessThan(o Value) Possibility

LessThan returns the possibility of v being less than o. o must be of type *EnumValue.

func (*EnumValue) Print 

func (v *EnumValue) Print(results *Results) string

Print returns a textual representation of the value.

func (*EnumValue) SetGreaterEqual 

func (v *EnumValue) SetGreaterEqual(o Value) Value

SetGreaterEqual returns a new value that represents the range of possible values in v that are greater than or equal to the lowest in o. o must be of type *EnumValue.

func (*EnumValue) SetGreaterThan 

func (v *EnumValue) SetGreaterThan(o Value) Value

SetGreaterThan returns a new value that represents the range of possible values in v that are greater than the lowest in o. o must be of type *EnumValue.

func (*EnumValue) SetLessEqual 

func (v *EnumValue) SetLessEqual(o Value) Value

SetLessEqual returns a new value that represents the range of possible values in v that are less than or equal to the highest in o. o must be of type *EnumValue.

func (*EnumValue) SetLessThan 

func (v *EnumValue) SetLessThan(o Value) Value

SetLessThan returns a new value that represents the range of possible values in v that are less than to the highest in o. o must be of type *EnumValue.

func (*EnumValue) String 

func (v *EnumValue) String() string

func (*EnumValue) Type 

func (v *EnumValue) Type() semantic.Type

Type returns the semantic type of the integer value represented by v.

func (*EnumValue) Union 

func (v *EnumValue) Union(o Value) Value

Union (∪) returns the values that are found in v or o. o must be of type *EnumValue.

func (*EnumValue) Valid 

func (v *EnumValue) Valid() bool

Valid returns true if there is any possibility of this value equaling any other.

type Labels 

type Labels map[uint64]string

Labels is a map of value to name.

func (Labels) Merge 

func (l Labels) Merge(o Labels)

Merge adds all the labels from o into l.

type MapValue 

type MapValue struct {
	// The map type.
	Map *semantic.Map

	// A map of keys to values. The inverse of ValueToKey.
	KeyToValue map[Value]Value

	// A map of values to keys. The inverse of KeyToValue.
	ValueToKey map[Value]Value
}

MapValue is an implementation of Value that represents a map variable. Map entries are stored in the KeyToValue and ValueToKey fields. These two maps are always kept in sync.

MapValue attempts to reduce the number of entries stored to a minimum to maintain decent performance while also trying to keep enough information to provide useful information (avoid tending towards unbounded keys and values).

All insertions and merges use the following method when merging the key value pair (k, v) into the map:

  • If m contains an equivalent key to k, then change the existing value to be a union of the existing value and v.
  • If m contains an equivalent value to v, then change the existing key to be a union of the existing key and k.
  • If m does not contain an equivalent key to k or an equivalent value to v then add a new entry to m.

func (*MapValue) Clone 

func (v *MapValue) Clone() Value

Clone returns a copy of v with a unique pointer.

func (*MapValue) ContainsKey 

func (v *MapValue) ContainsKey(key Value) Possibility

ContainsKey returns the possibility of the key existing in the map.

func (*MapValue) Difference 

func (v *MapValue) Difference(o Value) Value

Difference is not supported by MapValue and will panic if called.

func (*MapValue) Equals 

func (v *MapValue) Equals(o Value) Possibility

Equals returns False as maps do not support equality tests.

func (*MapValue) Equivalent 

func (v *MapValue) Equivalent(o Value) bool

Equivalent returns false as maps do not support equivalency tests.

func (*MapValue) Get 

func (v *MapValue) Get(s *scope, key Value) Value

Get return the union of all values that might have a key equal to key.

func (*MapValue) Intersect 

func (v *MapValue) Intersect(o Value) Value

Intersect is not supported by MapValue and will panic if called.

func (*MapValue) Print 

func (v *MapValue) Print(results *Results) string

Print returns a textual representation of the value.

func (*MapValue) Put 

func (v *MapValue) Put(key, val Value) *MapValue

Put returns a copy of v with the new mapping of key to val.

func (*MapValue) String 

func (v *MapValue) String() string

func (*MapValue) Type 

func (v *MapValue) Type() semantic.Type

Type returns the semantic map type of the value.

func (*MapValue) Union 

func (v *MapValue) Union(o Value) Value

Union (∪) returns a map value with all the keys of v and o mapped to their respective values. o must be of type *MapValue.

func (*MapValue) Valid 

func (v *MapValue) Valid() bool

Valid returns true.

type Possibility 

type Possibility int

Possibility is an enumerator of possibilites. 

const (
	// True represents a logical certanty or true.
	True Possibility = iota

	// Maybe represents the possibility of true or false.
	Maybe

	// False represents a logical certanty of false.
	False

	// Impossible represents a contradiction of certanties (for example
	// True ∩ False)
	Impossible
)

func (Possibility) And 

func (a Possibility) And(b Possibility) Possibility

And returns the logical-and of a and b.

func (Possibility) Difference 

func (a Possibility) Difference(b Possibility) Possibility

Difference returns the possibile for v that are not found in o.

func (Possibility) Equals 

func (a Possibility) Equals(b Possibility) Possibility

Equals returns the possibility of a equaling b.

func (Possibility) Intersect 

func (a Possibility) Intersect(b Possibility) Possibility

Intersect returns the intersection of possibile Possibilitys for a and b.

func (Possibility) MaybeFalse 

func (a Possibility) MaybeFalse() bool

MaybeFalse returns true iff a is False or Maybe.

func (Possibility) MaybeTrue 

func (a Possibility) MaybeTrue() bool

MaybeTrue returns true iff a is True or Maybe.

func (Possibility) Not 

func (a Possibility) Not() Possibility

Not returns the logical negation of a.

func (Possibility) Or 

func (a Possibility) Or(b Possibility) Possibility

Or returns the logical-or of a and b.

func (Possibility) Union 

func (a Possibility) Union(b Possibility) Possibility

Union returns the union of possibile Possibilitys for a and b.

type ReferenceValue 

type ReferenceValue struct {
	// The reference type.
	Ty semantic.Type
	// Unknown is the unknown value of the referenced type.
	Unknown Value
	// Assignments is the set of all assignments made to this value.
	Assignments map[*semantic.Create]struct{}
}

ReferenceValue is an implementation of Value that represents all the possible references of a reference value type. As references are an indirection, this value only holds the set of references not the set of referenced values. The actual referenced values are held by the scope. ReferenceValues uses *semantic.Create as the reference handles.

func (*ReferenceValue) Clone 

func (v *ReferenceValue) Clone() Value

Clone returns a copy of v with a unique pointer.

func (*ReferenceValue) Difference 

func (v *ReferenceValue) Difference(o Value) Value

Difference (\) returns the reference value with assignments that are found in v but not found in o. o must be of type *ReferenceValue.

func (*ReferenceValue) Equals 

func (v *ReferenceValue) Equals(o Value) Possibility

Equals returns the possibility of the reference value v being equal to o. o must be of type *ReferenceValue.

func (*ReferenceValue) Equivalent 

func (v *ReferenceValue) Equivalent(o Value) bool

Equivalent returns true iff v and o are equivalent. Unlike Equals() which returns the possibility of two values being equal, Equivalent() returns true iff the set of possible field values are exactly equal. o must be of type *ReferenceValue.

func (*ReferenceValue) Intersect 

func (v *ReferenceValue) Intersect(o Value) Value

Intersect (∩) returns the reference value the assignments that are found in both v and o. o must be of type *ReferenceValue.

func (*ReferenceValue) Print 

func (v *ReferenceValue) Print(results *Results) string

Print returns a textual representation of the value.

func (*ReferenceValue) String 

func (v *ReferenceValue) String() string

func (*ReferenceValue) Type 

func (v *ReferenceValue) Type() semantic.Type

Type returns the semantic reference type of the value.

func (*ReferenceValue) Union 

func (v *ReferenceValue) Union(o Value) Value

Union (∪) returns the reference value with all the assignments of v and o. o must be of type *ReferenceValue.

func (*ReferenceValue) Valid 

func (v *ReferenceValue) Valid() bool

Valid returns true.

type Relational 

type Relational interface {
	// GreaterThan returns the possibility of this value being greater than v.
	//
	// Examples:
	//     [5]  >  [5]  ->  False
	//    [1-3] > [2-5] ->  Maybe
	//    [1-3] > [4-6] ->  False
	//    [4-6] > [1-3] ->  True
	GreaterThan(v Value) Possibility

	// GreaterEqual returns the possibility of this value being greater or equal
	// to v.
	//
	// Examples:
	//     [5]  >=  [5]  ->  True
	//    [1-3] >= [2-5] ->  Maybe
	//    [1-3] >= [4-6] ->  False
	//    [4-6] >= [1-3] ->  True
	GreaterEqual(v Value) Possibility

	// LessThan returns the possibility of this value being less than v.
	//
	// Examples:
	//     [5]  <  [5]  ->  False
	//    [1-3] < [2-5] ->  Maybe
	//    [1-3] < [4-6] ->  True
	//    [4-6] < [1-3] ->  False
	LessThan(v Value) Possibility

	// LessEqual returns the possibility of this value being less than or equal
	// to v.
	//
	// Examples:
	//     [5]  <=  [5]  ->  True
	//    [1-3] <= [2-5] ->  Maybe
	//    [1-3] <= [4-6] ->  True
	//    [4-6] <= [1-3] ->  False
	LessEqual(v Value) Possibility
}

Relational is the interface implemented by values that can perform relational comparisons.

type Results 

type Results struct {
	// Unreachables is the list of unreachable blocks and statements.
	Unreachables []Unreachable
	// Globals is the map of semantic globals to their possible values.
	Globals map[*semantic.Global]Value
	// Parameters is the map of semantic parameters to their possible values.
	Parameters map[*semantic.Parameter]Value
	// Instances is the map of semantic create statements to the possible values
	// for those instances.
	Instances map[*semantic.Create]Value
}

Results holds the results of the static analysis.

func Analyze 

func Analyze(api *semantic.API, mappings *resolver.Mappings) *Results

Analyze performs static analysis on the API semantic tree.

type SetRelational 

type SetRelational interface {
	// SetGreaterThan returns a new value that represents the range of possible
	// values in this value that are greater than the lowest in v.
	//
	// Examples:
	//    [1-9] and  [5]  ->  [6-9]
	//    [1-9] and [2-5] ->  [3-9]
	//    [1-3] and [4-6] ->   [-]
	//    [4-6] and [1-3] ->  [4-6]
	SetGreaterThan(v Value) Value

	// SetGreaterEqual returns a new value that represents the range of possible
	// values in this value that are greater than or equal to the lowest in v.
	//
	// Examples:
	//    [1-9] and  [5]  ->  [5-9]
	//    [1-9] and [2-5] ->  [2-9]
	//    [1-3] and [4-6] ->   [-]
	//    [4-6] and [1-3] ->  [4-6]
	SetGreaterEqual(v Value) Value

	// SetLessThan returns a new value that represents the range of possible
	// values in this value that are less than to the highest in v.
	//
	// Examples:
	//    [1-9] and  [5]  ->  [1-4]
	//    [1-9] and [2-5] ->  [1-4]
	//    [1-3] and [4-6] ->  [1-3]
	//    [4-6] and [1-3] ->   [-]
	SetLessThan(v Value) Value

	// SetLessEqual returns a new value that represents the range of possible
	// values in this value that are less than or equal to the highest in v.
	//
	// Examples:
	//    [1-9] and  [5]  ->  [1-5]
	//    [1-9] and [2-5] ->  [1-5]
	//    [1-3] and [4-6] ->  [1-3]
	//    [4-6] and [1-3] ->   [-]
	SetLessEqual(v Value) Value
}

SetRelational is the interface implemented by values that produce new constrained values.

type UintValue 

type UintValue struct {
	Ty     *semantic.Builtin
	Ranges interval.U64SpanList
}

UintValue is an implementation of Value that represents all the possible values of an unsigned integer type. Although UintValue naturally represents unsigned integers, it can also be used to represent signed values by shifting the signed value into the unsigned range.

func (*UintValue) Clone 

func (v *UintValue) Clone() Value

Clone returns a copy of v with a unique pointer.

func (*UintValue) Difference 

func (v *UintValue) Difference(o Value) Value

Difference (\) returns the values that are found in v but not found in o. o must be of type *UintValue.

func (*UintValue) Equals 

func (v *UintValue) Equals(o Value) Possibility

Equals returns the possibility of v being equal to o. o must be of type *UintValue.

func (*UintValue) Equivalent 

func (v *UintValue) Equivalent(o Value) bool

Equivalent returns true iff v and o are equivalent. Unlike Equals() which returns the possibility of two values being equal, Equivalent() returns true iff the set of possible values are exactly equal. o must be of type *UintValue.

func (*UintValue) GreaterEqual 

func (v *UintValue) GreaterEqual(o Value) Possibility

GreaterEqual returns the possibility of v being greater or equal to o. o must be of type *UintValue.

func (*UintValue) GreaterThan 

func (v *UintValue) GreaterThan(o Value) Possibility

GreaterThan returns the possibility of v being greater than o. o must be of type *UintValue.

func (*UintValue) Intersect 

func (v *UintValue) Intersect(o Value) Value

Intersect (∩) returns the values that are found in both v and o. o must be of type *UintValue.

func (*UintValue) LessEqual 

func (v *UintValue) LessEqual(o Value) Possibility

LessEqual returns the possibility of v being less than or equal to o. o must be of type *UintValue.

func (*UintValue) LessThan 

func (v *UintValue) LessThan(o Value) Possibility

LessThan returns the possibility of v being less than o. o must be of type *UintValue.

func (*UintValue) Print 

func (v *UintValue) Print(results *Results) string

Print returns a textual representation of the value.

func (*UintValue) SetGreaterEqual 

func (v *UintValue) SetGreaterEqual(o Value) Value

SetGreaterEqual returns a new value that represents the range of possible values in v that are greater than or equal to the lowest in o. o must be of type *UintValue.

func (*UintValue) SetGreaterThan 

func (v *UintValue) SetGreaterThan(o Value) Value

SetGreaterThan returns a new value that represents the range of possible values in v that are greater than the lowest in o. o must be of type *UintValue.

func (*UintValue) SetLessEqual 

func (v *UintValue) SetLessEqual(o Value) Value

SetLessEqual returns a new value that represents the range of possible values in v that are less than or equal to the highest in o. o must be of type *UintValue.

func (*UintValue) SetLessThan 

func (v *UintValue) SetLessThan(o Value) Value

SetLessThan returns a new value that represents the range of possible values in v that are less than to the highest in o. o must be of type *UintValue.

func (*UintValue) String 

func (v *UintValue) String() string

func (*UintValue) Type 

func (v *UintValue) Type() semantic.Type

Type returns the semantic type of the integer value represented by v.

func (*UintValue) Union 

func (v *UintValue) Union(o Value) Value

Union (∪) returns the values that are found in v or o. o must be of type *UintValue.

func (*UintValue) Valid 

func (v *UintValue) Valid() bool

Valid returns true if there is any possibility of this value equaling any other.

type Unreachable 

type Unreachable struct {
	At   parse.Node
	Node semantic.Node
}

Unreachable represents an unreachable block or statement.

type UntrackedValue 

type UntrackedValue struct {
	// Ty is the type of the untracked value.
	Ty semantic.Type
}

UntrackedValue is an implementation of Value used for types that are not tracked by static analysis.

func (*UntrackedValue) Clone 

func (v *UntrackedValue) Clone() Value

Clone returns a new pointer to a UntrackedValue.

func (*UntrackedValue) Difference 

func (v *UntrackedValue) Difference(o Value) Value

Difference returns a new pointer to a UntrackedValue.

func (*UntrackedValue) Equals 

func (v *UntrackedValue) Equals(Value) Possibility

Equals returns Maybe.

func (*UntrackedValue) Equivalent 

func (v *UntrackedValue) Equivalent(Value) bool

Equivalent returns true.

func (*UntrackedValue) Intersect 

func (v *UntrackedValue) Intersect(o Value) Value

Intersect returns a new pointer to a UntrackedValue.

func (*UntrackedValue) Print 

func (v *UntrackedValue) Print(*Results) string

Print returns "<untracked>"

func (*UntrackedValue) Type 

func (v *UntrackedValue) Type() semantic.Type

Type returns the semantic type of the untracked value.

func (*UntrackedValue) Union 

func (v *UntrackedValue) Union(o Value) Value

Union returns a new pointer to a UntrackedValue.

func (*UntrackedValue) Valid 

func (v *UntrackedValue) Valid() bool

Valid returns true.

type Value 

type Value interface {
	// Print returns a textual representation of the value.
	Print(results *Results) string

	// Type returns the semantic type of the value.
	Type() semantic.Type

	// Equivalent returns true iff this and v are equivalent.
	// Unlike Equals() which returns the possibility of two values being equal,
	// Equivalent() returns true iff the set of possible values are exactly
	// equal.
	//
	// Examples:
	//     [5]  and  [5]  are equivalent.
	//    [1-5] and [1-5] are equivalent.
	//    [1-3] and  [1]  are not equivalent.
	//     [1]  and  [2]  are not equivalent.
	Equivalent(v Value) bool

	// Equals returns the possibility of this value being equal to v.
	// If this and v have go-equality then Equals() will return True.
	//
	// Examples:
	//     [5]  and  [5]  returns True.
	//    [1-5] and [1-5] returns Maybe.
	//    [1-3] and [5-7] returns False.
	//     [1]  and  [2]  returns False.
	Equals(v Value) Possibility

	// Valid returns true if there is any possibility of this value equaling
	// any other.
	Valid() bool

	// Clone returns a copy of this value with a unique instance.
	// The cloned value will not have go-equality with this and so will not be
	// considered to have algerbraic equality.
	Clone() Value

	// Union (∪) returns the values that are found in this or v.
	//
	// Examples:
	//     [5]  ∪  [5]  ->      [5]
	//    [1-3] ∪ [2-5] ->     [1-5]
	//    [1-3] ∪ [4-6] ->  [1-3] [4-6]
	//     [1]  ∪  [3]  ->   [1]   [3]
	Union(v Value) Value

	// Intersect (∩) returns the values that are found in both this and v.
	//
	// Examples:
	//     [5]  ∩  [5]  ->   [5]
	//    [1-3] ∩ [2-5] ->  [2-3]
	//    [1-3] ∩ [4-6] ->   [-]
	//     [1]  ∩  [3]  ->   [-]
	Intersect(v Value) Value

	// Difference (\) returns the values that are found in this but not found in v.
	//
	// Examples:
	//     [5]  \  [5]  ->   [-]
	//    [1-3] \ [2-5] ->   [1]
	//    [1-3] \ [4-6] ->  [1-3]
	//     [1]  \  [3]  ->   [1]
	Difference(v Value) Value
}

Value represents the data flow analysis representation of a variable or expression. Unlike the value of a variable or expression at runtime, the static analysis representation of a value usually represents a range of possible runtime values. As such many of the value implementations are backed by some sort of set.

Values are immutable. All transformation methods on a value should return a new value.

Values that have go-equality can be considered to have algerbraic equality, even if that Value represents a wide range of possible runtime values.

func UnionOf 

func UnionOf(vals ...Value) Value

UnionOf returns the union of all the values in the slice vals. nil values in the slice are ignored.

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