aws/endpoints

Smithy endpoint rule set evaluator.

Implements the runtime side of the Smithy rules engine: parses a endpoint-rule-set.json document, then walks it against a set of parameter values and produces a concrete endpoint URL.

Supported features:

Rule types: endpoint, error, tree
Parameters: String, Boolean, with default and required
Templates: {Name} and {Var#field} (and nested {Var#a#b})
Built-in functions: isSet, not, booleanEquals, stringEquals, getAttr, substring, uriEncode, parseURL, isValidHostLabel, aws.partition, aws.parseArn, aws.isVirtualHostableS3Bucket
Common partition data hardcoded (aws, aws-cn, aws-us-gov). For a richer table, vendor partitions.json and replace partition_for/1.

Not implemented (rare; flagged at evaluation time as Unsupported):

StringArray parameter type (used by a few new services)

Codegen at milestone 7 will use this evaluator at compile time to emit per-service endpoint resolvers; the runtime fallback path here keeps hand-written services working before any code is generated.

Types

Condition

</>

pub type Condition {
  Condition(expr: Expr, assign: option.Option(String))
}

Constructors

Condition(expr: Expr, assign: option.Option(String))

Endpoint

</>

pub type Endpoint {
  Endpoint(url: String, headers: dict.Dict(String, List(String)))
}

Constructors

Endpoint(url: String, headers: dict.Dict(String, List(String)))

EndpointSpec

</>

pub type EndpointSpec {
  EndpointSpec(url: Expr, headers: dict.Dict(String, List(Expr)))
}

Constructors

EndpointSpec(url: Expr, headers: dict.Dict(String, List(Expr)))

Expr

</>

pub type Expr {
  Ref(name: String)
  TemplateExpr(parts: List(TemplatePart))
  BoolLit(value: Bool)
  IntLit(value: Int)
  FnCall(name: String, args: List(Expr))
}

Constructors

```
Ref(name: String)
```
Reference to a parameter or assigned variable.
```
TemplateExpr(parts: List(TemplatePart))
```
String literal that may contain {Name} and {Var#field} interpolations.
```
BoolLit(value: Bool)
```
```
IntLit(value: Int)
```
Integer literal — substring(input, 0, 3, false) and similar.
```
FnCall(name: String, args: List(Expr))
```
Builtin function call. The function name is the raw “fn” value (e.g. "isSet", "aws.partition").

Parameter

</>

pub type Parameter {
  Parameter(
    type_: ParamType,
    required: Bool,
    default: option.Option(Value),
    builtin: option.Option(String),
  )
}

Constructors

Parameter(
  type_: ParamType,
  required: Bool,
  default: option.Option(Value),
  builtin: option.Option(String),
)

Params

</>

Parameter values supplied to evaluation. Keys are parameter names.

pub type Params =
  dict.Dict(String, Value)

ResolveError

</>

pub type ResolveError {
  RuleError(message: String)
  NoMatch
  InvalidRuleSet(reason: String)
  Unsupported(reason: String)
  MissingParameter(name: String)
  RequiredParameterMissing(name: String)
}

Constructors

```
RuleError(message: String)
```
A rule’s error branch fired with this message.
```
NoMatch
```
No rule matched (the rule set is supposed to always end in either an endpoint or an error rule, so this normally indicates a bad rule set).
```
InvalidRuleSet(reason: String)
```
JSON could not be parsed into a RuleSet.
```
Unsupported(reason: String)
```
Evaluation tripped over an unimplemented function or a parameter type the evaluator doesn’t support yet.
```
MissingParameter(name: String)
```
A parameter referenced by a rule wasn’t supplied and has no default.
```
RequiredParameterMissing(name: String)
```
A required parameter is unset.

Rule

</>

pub type Rule {
  EndpointRule(
    conditions: List(Condition),
    endpoint: EndpointSpec,
  )
  ErrorRule(conditions: List(Condition), message: Expr)
  TreeRule(conditions: List(Condition), rules: List(Rule))
}

Constructors

EndpointRule(conditions: List(Condition), endpoint: EndpointSpec)

ErrorRule(conditions: List(Condition), message: Expr)

TreeRule(conditions: List(Condition), rules: List(Rule))

RuleSet

</>

pub type RuleSet {
  RuleSet(
    parameters: dict.Dict(String, Parameter),
    rules: List(Rule),
  )
}

Constructors

RuleSet(
  parameters: dict.Dict(String, Parameter),
  rules: List(Rule),
)

TemplatePart

</>

pub type TemplatePart {
  Static(text: String)
  Interp(path: List(String))
}

Constructors

```
Static(text: String)
```
```
Interp(path: List(String))
```

Runtime values that flow through evaluation: literals plus the record shape that aws.partition returns plus the list shape used for stringArray parameters and the resourceId field of a parsed ARN.

pub type Value {
  StringVal(String)
  BoolVal(Bool)
  RecordVal(dict.Dict(String, Value))
  ListVal(List(Value))
  EmptyVal
}

Constructors

```
StringVal(String)
```
```
BoolVal(Bool)
```
```
RecordVal(dict.Dict(String, Value))
```
```
ListVal(List(Value))
```
```
EmptyVal
```

Values

params_from

</>

pub fn params_from(
  strings strings: List(#(String, String)),
  bools bools: List(#(String, Bool)),
) -> dict.Dict(String, Value)

Convenience constructor: build a Params map from string and boolean pairs. The most common shape (Region + UseFips + UseDualStack).

parse_rule_set

</>

pub fn parse_rule_set(
  json_text: String,
) -> Result(RuleSet, ResolveError)

Parse a rule set JSON document (as a Gleam string) into the internal AST. The result is reusable across many resolve calls with different params.

resolve

</>

pub fn resolve(
  rule_set: RuleSet,
  params: dict.Dict(String, Value),
) -> Result(Endpoint, ResolveError)

Walk the rule set with the given parameters. Returns the first matching endpoint, the first matching error rule (surfaced as RuleError), or NoMatch if every rule’s conditions failed.