github.com/solo-io/cue@v0.4.7/internal/core/adt/doc.go (about)

     1  // Copyright 2020 CUE Authors
     2  //
     3  // Licensed under the Apache License, Version 2.0 (the "License");
     4  // you may not use this file except in compliance with the License.
     5  // You may obtain a copy of the License at
     6  //
     7  //     http://www.apache.org/licenses/LICENSE-2.0
     8  //
     9  // Unless required by applicable law or agreed to in writing, software
    10  // distributed under the License is distributed on an "AS IS" BASIS,
    11  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    12  // See the License for the specific language governing permissions and
    13  // limitations under the License.
    14  
    15  // Package adt represents partially and fully evaluated CUE types.
    16  //
    17  // This package introduces several categories of types that indicate some set of
    18  // values that may be used in a certain situation. Concrete types may belong to
    19  // multiple categories.
    20  //
    21  //
    22  // Abstract Types
    23  //
    24  // The following types describe the a place where a value may be used:
    25  //
    26  //    Decl       a value than can be used as a StructLit element.
    27  //    Elem       a value than can be used as a ListLit element.
    28  //    Expr       represents an Expr in the CUE grammar.
    29  //    Value      a fully evaluated value that has no references (except for
    30  //               children in composite values).
    31  //    Node       any of the above values.
    32  //
    33  // The following types categorize nodes by function:
    34  //
    35  //    Resolver   a reference to position in the result tree.
    36  //    Evaluator  evaluates to 1 value.
    37  //    Yielder    evaluates to 0 or more values.
    38  //    Validator  validates another value.
    39  //
    40  //
    41  // Reference resolution algorithm
    42  //
    43  // A Resolver is resolved within the context of an Environment. In CUE, a
    44  // reference is evaluated by substituting it with a copy of the value to which
    45  // it refers. If the copied value itself contains references we can distinguish
    46  // two different cases. References that refer to values within the copied
    47  // reference (not regarding selectors) will henceforth point to the copied node.
    48  // References that point to outside the referened value will keep referring to
    49  // their original value.
    50  //
    51  //      a: b: {
    52  //        c: int
    53  //        d: c
    54  //        e: f
    55  //      }
    56  //      f: 4
    57  //      g: a.b { // d.c points to inside the referred value, e.f, not.
    58  //        c: 3
    59  //      }
    60  //
    61  // The implementation doesn't actually copy referred values, but rather resolves
    62  // references with the aid of an Environment. During compile time, each
    63  // references is associated with the label and a number indicating in which
    64  // parent scope (offset from the current) this label needs to be looked up. An
    65  // Environment keeps track of the point at which a value was referenced,
    66  // providing enough information to look up the labeled value. This Environment
    67  // is the identical for all references within a fields conjunct. Often, an
    68  // Environment can even be shared among conjuncts.
    69  //
    70  //
    71  // Values
    72  //
    73  // Values are fully evaluated expressions. As this means that all references
    74  // will have been eliminated, Values are fully defined without the need for an
    75  // Environment. Additionally, Values represent a fully evaluated form, stripped
    76  // of any comprehensions, optional fields or embeddings.
    77  //
    78  package adt