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docs: New documentation about the "Refinements" concept
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# Value Refinements | ||
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_Refinements_ are dynamic annotations associated with unknown values that | ||
each shrink the range of possible values futher than can be represented by | ||
type constraint alone. | ||
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When an unknown value is refined, it allows certain operations against that | ||
unknown value to produce a known result, and allows some operations to fail | ||
earlier than they would with a fully-unknown value by detecting that a valid | ||
result is impossible using just the refinement information. | ||
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Refinements always _shrink_ the range of an unknown value, and never grow it. | ||
That makes it valid for some operations to ignore refinements and just treat | ||
an unknown value as representing any possible value of its type constraint, | ||
which is important to avoid burdening all downstream callers of `cty` from | ||
handling all refinements and from immediately adding support for new kinds of | ||
refinement if this model gets extended in future releases. | ||
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However, note that `Value.RawEquals` _does_ take into account refinements, so | ||
any tests that assert against the exact final value of an operation may need | ||
to be updated after adopting a new version of `cty` which makes increased use | ||
of refinements. `Value.RawEquals` is not intended as part of the _user model_ | ||
of `cty` and so this should not negatively impact the end-user-visible behavior | ||
of an application using `cty`, although of course they might benefit from | ||
more specific results from operations that can now take refinements into | ||
account. | ||
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## How to refine a value | ||
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You can derive a more refined value from a less refined value by using the | ||
`Value.Refine` method to obtain a _refinement builder_, which uses the | ||
builder pattern to construct a new value with one or more extra refinements. | ||
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```go | ||
val := cty.UnknownVal(cty.String).Refine(). | ||
NotNull(). | ||
StringPrefix("https://"). | ||
NewValue() | ||
``` | ||
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The above snippet would produce a refined local value whose range is limited | ||
only to non-null strings which start with the prefix `"https://"`. This | ||
information can, in theory, allow `val.Equals(cty.NullVal(cty.String))` to | ||
return `cty.False` rather than `cty.UnknownVal(cty.Bool)`, and allow a prefix | ||
match against the string to return a known result. | ||
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In practice not all operations against unknown values can make full use of | ||
unknown value refinements, but hopefully the coverage will increase over time. | ||
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Only unknown values can have refinements, because known values are already | ||
refined by their concrete value: simple values like `cty.Zero` are constrained | ||
to exactly one value, while some values like `cty.ListValEmpty(cty.DynamicPseudoType)` | ||
represent a set of possible values -- all empty lists of any element type, in | ||
this case. | ||
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However, the `Refine` operation _is_ also supported for known values and in that | ||
case acts as a self-checking assertion that the known value does actually | ||
meet the requirements. If you write your codepaths to unconditionally assign | ||
refinements regardless of whether the value is known then your code will | ||
self-check and raise a panic if the final known value doesn't match the | ||
previously-promised refinements. | ||
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A similar rule applies to applying new refinements already-refined values: it's | ||
fine to describe a less specific refinement, which will therefore be ignored | ||
because it adds no new information. It's an application bug to describe a | ||
contradictory refinement, such as a new string prefix that doesn't match one | ||
previously assigned. | ||
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## Value ranges | ||
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The `Refine()` method described above constructs a value with refinements. To | ||
access the information from those refinements, use the `Value.Range` method to | ||
obtain a `cty.ValueRange` object, which describes a superset of all of the | ||
values that a particular value could have. | ||
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For example, you can use `val.Range().DefinitelyNotNull()` to test whether a | ||
particular value is guaranteed to be non-null once it is finally known. This | ||
again works for both known and unknown values, so e.g. | ||
`cty.StringVal("foo").Range().DefinitelyNotNull()` returns `true` because | ||
a known, non-null string value is _definitely not null_. | ||
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When writing operations that depend only on information that can be determined | ||
from refinements it's valid to depend exclusively on `Value.Range` and rely on | ||
the fact that the range of an already-known value is just a very narrow range | ||
that covers only what that specific value covers. | ||
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The model of value ranges is imprecise, though: it's limited only to information | ||
we can track for unknown values through refinements. Many operations will still | ||
need a special codepath to handle the unknown case vs. the known case so they | ||
can take into account the additional detail from the exact value once known. | ||
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## Available Refinements | ||
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The set of possible refinement types might grow over time, but the initial set | ||
is focused on a narrow set of possibilities that seems likely to allow a number | ||
of other operations to either produce known results from unknown input or to | ||
rule that particular input is invalid despite not yet being known. | ||
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The most notable restriction on refinements is that the available refinements | ||
vary depending on the type constraint of the value being refined. | ||
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The least flexible case is `cty.DynamicVal` -- an unknown value of an unknown | ||
type -- which is the one value that cannot be refined at all and will cause | ||
a panic if you try. This is a pragmatic compromise for backward compatibility: | ||
existing callers use patterns like `val == cty.DynamicVal` to test for this | ||
specific special value, and any refinements of that value would make it no | ||
longer equal. | ||
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Unknown values of built-in exact types, and also unknown values whose type | ||
_kind_ is constrained even if the element/attribute types are not, can at | ||
least be refined as being non-null, and because that is a common situation | ||
there is a shorthand for it which avoids using the builder pattern: | ||
`val.RefineNotNull()`. | ||
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All other possible refinements are type-constraint-specific: | ||
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* `cty.String` | ||
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For strings we can refine a known prefix of the string, which is intended | ||
for situations where the string represents some microsyntax with a | ||
known prefix, such as a URL of a particular known scheme. | ||
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* `.StringPrefix(string)` specifies a known prefix of the final string. | ||
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By default an unknown string has no known prefix, which is the same | ||
as the prefix being the empty string. | ||
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Because `cty`'s model of strings is a sequence of Unicode grapheme | ||
clusters, `.StringPrefix` will quietly disregard trailing Unicode | ||
code units of the given prefix that might combine with other code | ||
units to form a new combined grapheme. This is a good safe default | ||
behavior for situations where the remainder of the string is under | ||
end-user control and might begin with combining diacritics or | ||
emoji variation sequences. Applications should not rely on the | ||
details of this heuristic because it may become more precise in | ||
later releases. | ||
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* `.StringPrefixFull(string)` is like `.StringPrefix` but does not trim | ||
possibly-combining code units from the end of the given string. | ||
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Applications must use this with care, making sure that they control | ||
the final string enough to guarantee that the subsequent additional | ||
code units will never combine with any characters in the given prefix. | ||
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* `cty.Number` | ||
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For numbers we can refine both the lower and upper bound of possible values, | ||
with each boundary being either inclusive or exclusive. | ||
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* `.NumberRangeLowerBound(cty.Value, bool)` refines the lower bound of | ||
possible values for an unknown number. The boolean argument represents | ||
whether the bound is _inclusive_. | ||
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The given value must be a non-null `cty.Number` value. An unrefined | ||
number effectively has a lower bound of `(cty.NegativeInfinity, true)`. | ||
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* `.NumberRangeUpperBound(cty.Value, bool)` refines the upper bound of | ||
possible values for an unknown number. The boolean argument represents | ||
whether the bound is _inclusive_. | ||
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The given value must be a non-null `cty.Number` value. An unrefined | ||
number effectively has an upper bound of `(cty.PositiveInfinity, true)`. | ||
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* `.NumberRangeInclusive(min, max cty.Value)` is a helper wrapper around | ||
the previous two methods that declares both an upper and lower bound | ||
at the same time, while specifying that both are inclusive bounds. | ||
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* `cty.List`, `cty.Set`, and `cty.Map` types | ||
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For all collection types we can refine the lower and upper bound of the | ||
length of the collection. The boundaries on length are always inclusive | ||
and are integers, because it isn't possible to have a fraction of an | ||
element. | ||
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* `.CollectionLengthLowerBound(int)` refines the lower bound of possible | ||
lengths for an unknown collection. | ||
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An unrefined collection effectively has a lower bound of zero, because | ||
it's not possible for a collection to have a negative length. | ||
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* `.CollectionLengthUpperBound(int)` refines the upper bound of possible | ||
lengths for an unknown collection. | ||
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An unrefined collection has an upper bound that matches the largest | ||
valid Go slice index on the current platform, because `cty`'s | ||
collections are implemented in terms of Go's collection types. | ||
However, applications should typically not expose that specific value | ||
to users (it's an implementation detail) and should instead present | ||
the maximum value as an unconstrained length. | ||
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* `.CollectionLength(int)` is a shorthand that refines both the lower and | ||
upper bounds to the same value. This is a helpful requirement to make | ||
whenever possible because it will often allow the final value to be | ||
a known collection with unknown elements, as described in | ||
[Refinement Value Collapse](#refinement-value-collapse). | ||
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Some built-in operations will automatically take into account refinements from | ||
their input operands and propagate them in a suitable way to the result. | ||
However, that is not a guarantee for all operations and so should be treated | ||
as a "best effort" behavior which will hopefully become more precise in future | ||
versions. | ||
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Behaviors implemented in downstream applications, such as custom functions | ||
using [the function system](functions.md), might also take into account | ||
refinements. If they do their work using only _operation methods_ on `Value` | ||
then the handling of refinements might come for free. If they do work using | ||
_integration methods_ instead then they will need to explicitly handle | ||
refinements if desired. If they don't then by default the result from an | ||
unknown input will be a totally-unrefined unknown value, though will hopefully | ||
still have a useful type constraint. | ||
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## Refinement Value Collapse | ||
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For some kinds of refinement it's possible to constrain the range so much that | ||
only one possible value remains. In that case, the `.NewValue()` method of the | ||
refinement builder might return a known value instead of an unknown value. | ||
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For example, if the lower bound and upper bound of a collection's length are | ||
equal then the length of the collection is effectively known. For some lengths | ||
of some collection kinds the refinement can collapse into a known collection | ||
containing unknown values. For example, an unknown list that's known to have | ||
exactly two values can be represented equivalently as a known list of length | ||
two where both elements are unknown themselves. | ||
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The exact details of how refinement collapse is decided might change in future | ||
versions, but only in ways that can make results "more known". It would be a | ||
breaking change to weaken a rule to produce unknown values in more cases, so | ||
that kind of change would be reserved only for fixing an important bug or | ||
design error. | ||
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## Refinements are Dynamic Only | ||
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Refinements belong to unknown values rather than to type constraints, and so | ||
refining an unknown value does not change its type constraint. | ||
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This design is a tradeoff: making the refinements dynamic and implicit means | ||
that it's possible to add more detailed refinements over type without making | ||
breaking changes to explicit type information, but the downside is that | ||
it isn't possible to represent refinements in any situation that is only | ||
aware of types. | ||
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For example, it isn't currently possible to represent the idea of an unknown | ||
map whose elements each have a further refinement applied, because the | ||
refinements apply to the map itself and there are not yet any specific element | ||
values for the element refinements to attach to. | ||
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(It would be possible in theory to allow refining an unknown collection with | ||
meta-refinements about its hypothetical elements, but that is not currently | ||
supported because it would mean that refinements would need to be resolved | ||
recursively and that would be considerably more complex and expensive than | ||
the current single-value-only refinements structure.) | ||
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## Refinements Under Serialization | ||
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Refinements are intentionally designed so that they only constrain the range | ||
of an unknown value, and never expand it. This means that it should typically | ||
be safe to discard refinements in situations like serialization where there | ||
may not be any way to represent the refinements. After decoding the unknown | ||
value now has a wider range but it should still be a superset of the true | ||
range of the value. This is an example of the general rule that no operation | ||
on an unknown value is _guaranteed_ to fully preserve the input refinements | ||
or to consider them when calculating the result. | ||
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The official MessagePack serialization in particular does have some support | ||
for retaining approximations of refinements as part of its serialization of | ||
unknown values, using a MessagePack extension value. Some detail may still | ||
be lost under round-tripping but the output range should always be a superset | ||
of the input range. As long as both the serializer and deserializer are using | ||
the `cty/msgpack` sub-package unknown values will propagate automatically | ||
without any additional caller effort. | ||
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## Relationship to "Marks" | ||
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The idea of annotating a value with additional information has some overlap | ||
with the concept of [Marks](marks.md). However, the two have different purposes | ||
and so different design details and tradeoffs. | ||
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Marks should typically be used for additional information that is independent | ||
of the specific type and value, such as marking a value as having come from | ||
a sensitive location. The marking then propagates to all results from operations | ||
on that value, usually without changing the behavior of that operation. In a | ||
sense the mark represents the _origin_ of the value rather than the value | ||
itself. | ||
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Refinements are instead directly part of the value. By reducing the possible | ||
range of an unknown value placeholder, other downstream operations can in turn | ||
produce a more refined result, or possibly even a known result from unknown | ||
inputs. Refinements do not naively propagate from one value to the next, but | ||
some operations will use the refinements of their operands to calculate a new | ||
set of refiments for their result, with the rules varying on a case-by-case | ||
basis depending on what calculation the operation represents. |