vendor/github.com/zclconf/go-cty/cty/path.go

   1 package cty
   2
   3 import (
   4         "errors"
   5         "fmt"
   6 )
   7
   8 // A Path is a sequence of operations to locate a nested value within a
   9 // data structure.
  10 //
  11 // The empty Path represents the given item. Any PathSteps within represent
  12 // taking a single step down into a data structure.
  13 //
  14 // Path has some convenience methods for gradually constructing a path,
  15 // but callers can also feel free to just produce a slice of PathStep manually
  16 // and convert to this type, which may be more appropriate in environments
  17 // where memory pressure is a concern.
  18 //
  19 // Although a Path is technically mutable, by convention callers should not
  20 // mutate a path once it has been built and passed to some other subsystem.
  21 // Instead, use Copy and then mutate the copy before using it.
  22 type Path []PathStep
  23
  24 // PathStep represents a single step down into a data structure, as part
  25 // of a Path. PathStep is a closed interface, meaning that the only
  26 // permitted implementations are those within this package.
  27 type PathStep interface {
  28         pathStepSigil() pathStepImpl
  29         Apply(Value) (Value, error)
  30 }
  31
  32 // embed pathImpl into a struct to declare it a PathStep implementation
  33 type pathStepImpl struct{}
  34
  35 func (p pathStepImpl) pathStepSigil() pathStepImpl {
  36         return p
  37 }
  38
  39 // Index returns a new Path that is the reciever with an IndexStep appended
  40 // to the end.
  41 //
  42 // This is provided as a convenient way to construct paths, but each call
  43 // will create garbage so it should not be used where memory pressure is a
  44 // concern.
  45 func (p Path) Index(v Value) Path {
  46         ret := make(Path, len(p)+1)
  47         copy(ret, p)
  48         ret[len(p)] = IndexStep{
  49                 Key: v,
  50         }
  51         return ret
  52 }
  53
  54 // IndexPath is a convenience method to start a new Path with an IndexStep.
  55 func IndexPath(v Value) Path {
  56         return Path{}.Index(v)
  57 }
  58
  59 // GetAttr returns a new Path that is the reciever with a GetAttrStep appended
  60 // to the end.
  61 //
  62 // This is provided as a convenient way to construct paths, but each call
  63 // will create garbage so it should not be used where memory pressure is a
  64 // concern.
  65 func (p Path) GetAttr(name string) Path {
  66         ret := make(Path, len(p)+1)
  67         copy(ret, p)
  68         ret[len(p)] = GetAttrStep{
  69                 Name: name,
  70         }
  71         return ret
  72 }
  73
  74 // GetAttrPath is a convenience method to start a new Path with a GetAttrStep.
  75 func GetAttrPath(name string) Path {
  76         return Path{}.GetAttr(name)
  77 }
  78
  79 // Apply applies each of the steps in turn to successive values starting with
  80 // the given value, and returns the result. If any step returns an error,
  81 // the whole operation returns an error.
  82 func (p Path) Apply(val Value) (Value, error) {
  83         var err error
  84         for i, step := range p {
  85                 val, err = step.Apply(val)
  86                 if err != nil {
  87                         return NilVal, fmt.Errorf("at step %d: %s", i, err)
  88                 }
  89         }
  90         return val, nil
  91 }
  92
  93 // LastStep applies the given path up to the last step and then returns
  94 // the resulting value and the final step.
  95 //
  96 // This is useful when dealing with assignment operations, since in that
  97 // case the *value* of the last step is not important (and may not, in fact,
  98 // present at all) and we care only about its location.
  99 //
 100 // Since LastStep applies all steps except the last, it will return errors
 101 // for those steps in the same way as Apply does.
 102 //
 103 // If the path has *no* steps then the returned PathStep will be nil,
 104 // representing that any operation should be applied directly to the
 105 // given value.
 106 func (p Path) LastStep(val Value) (Value, PathStep, error) {
 107         var err error
 108
 109         if len(p) == 0 {
 110                 return val, nil, nil
 111         }
 112
 113         journey := p[:len(p)-1]
 114         val, err = journey.Apply(val)
 115         if err != nil {
 116                 return NilVal, nil, err
 117         }
 118         return val, p[len(p)-1], nil
 119 }
 120
 121 // Copy makes a shallow copy of the receiver. Often when paths are passed to
 122 // caller code they come with the constraint that they are valid only until
 123 // the caller returns, due to how they are constructed internally. Callers
 124 // can use Copy to conveniently produce a copy of the value that _they_ control
 125 // the validity of.
 126 func (p Path) Copy() Path {
 127         ret := make(Path, len(p))
 128         copy(ret, p)
 129         return ret
 130 }
 131
 132 // IndexStep is a Step implementation representing applying the index operation
 133 // to a value, which must be of either a list, map, or set type.
 134 //
 135 // When describing a path through a *type* rather than a concrete value,
 136 // the Key may be an unknown value, indicating that the step applies to
 137 // *any* key of the given type.
 138 //
 139 // When indexing into a set, the Key is actually the element being accessed
 140 // itself, since in sets elements are their own identity.
 141 type IndexStep struct {
 142         pathStepImpl
 143         Key Value
 144 }
 145
 146 // Apply returns the value resulting from indexing the given value with
 147 // our key value.
 148 func (s IndexStep) Apply(val Value) (Value, error) {
 149         if val == NilVal || val.IsNull() {
 150                 return NilVal, errors.New("cannot index a null value")
 151         }
 152
 153         switch s.Key.Type() {
 154         case Number:
 155                 if !(val.Type().IsListType() || val.Type().IsTupleType()) {
 156                         return NilVal, errors.New("not a list type")
 157                 }
 158         case String:
 159                 if !val.Type().IsMapType() {
 160                         return NilVal, errors.New("not a map type")
 161                 }
 162         default:
 163                 return NilVal, errors.New("key value not number or string")
 164         }
 165
 166         has := val.HasIndex(s.Key)
 167         if !has.IsKnown() {
 168                 return UnknownVal(val.Type().ElementType()), nil
 169         }
 170         if !has.True() {
 171                 return NilVal, errors.New("value does not have given index key")
 172         }
 173
 174         return val.Index(s.Key), nil
 175 }
 176
 177 func (s IndexStep) GoString() string {
 178         return fmt.Sprintf("cty.IndexStep{Key:%#v}", s.Key)
 179 }
 180
 181 // GetAttrStep is a Step implementation representing retrieving an attribute
 182 // from a value, which must be of an object type.
 183 type GetAttrStep struct {
 184         pathStepImpl
 185         Name string
 186 }
 187
 188 // Apply returns the value of our named attribute from the given value, which
 189 // must be of an object type that has a value of that name.
 190 func (s GetAttrStep) Apply(val Value) (Value, error) {
 191         if val == NilVal || val.IsNull() {
 192                 return NilVal, errors.New("cannot access attributes on a null value")
 193         }
 194
 195         if !val.Type().IsObjectType() {
 196                 return NilVal, errors.New("not an object type")
 197         }
 198
 199         if !val.Type().HasAttribute(s.Name) {
 200                 return NilVal, fmt.Errorf("object has no attribute %q", s.Name)
 201         }
 202
 203         return val.GetAttr(s.Name), nil
 204 }
 205
 206 func (s GetAttrStep) GoString() string {
 207         return fmt.Sprintf("cty.GetAttrStep{Name:%q}", s.Name)
 208 }