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Rib- is a feature in Golem OSS, that enables users to write programs capable of manipulating worker responses, which are WebAssembly (WASM) values. Currently, Rib language is designed to function seamlessly with the worker-bridge of Golem. Refer to worker-bridge documentation, where we extensively use Rib.

Purpose of Rib

Golem lets you invoke functions on workers directly. However, sometimes you want to put a specific API in front of those workers, such as HTTP, gRPC, or GraphQL. Yet, each of these different protocols has certain expectations around the structure of requests and responses. Rib lets you adapt inputs and outputs without having to change the definitions of functions on your workers. It's the glue that lets you build any type of API atop your workers.

Currently, the Worker Bridge allows you to import OpenAPI specification with HTTP endpoints (or define API Definition in worker-bridge's format), each having its own worker-binding. Refer worker-bridge documentation for more detail.

A worker-binding generally includes the worker name, the name of function in the worker to be invoked, it's function parameters, and the response mapping, which tells how to change the worker response to the API response. The Rib language is used in many places in this context. Example: You can utilize this language to construct the worker name, which can be based on the request object, a constant literal, or a combination of both. For example: foo-${request.body.user}.

For further guidance on using Rib language in conjunction with the Worker Bridge, refer to the Worker Bridge documentation.

With that, let's explain the grammar of this language and a few examples and corner cases

For the most representation of the values are making use of WASM syntax.

Rib Grammar

rib-expr below defines the various possibilities of expressions that can be used in Rib. Consider this as a reference. Probably all you need is to look at the examples further given below.

rib-expr            ::= request
                   | let_binding
                   | select_field
                   | select_index
                   | sequence
                   | record
                   | tuple
                   | literal
                   | number
                   | flags
                   | variable
                   | boolean
                   | concat
                   | multiple
                   | not
                   | greater_than
                   | greater_than_or_equal_to
                   | less_than_or_equal_to
                   | equal_to
                   | less_than
                   | conditional
                   | pattern_match
                   | option
                   | result
                   | function-call
let_binding       ::= "let" variable '=' rib-expr ';'
select_field       ::= request_field | worker_field | (record | variable)'.'variable_sequence
worker_field       := "worker.response" | "worker.response." variable_sequence
request_field      ::= request_body | request_path | request_header
request_body      ::= "request.body" | "request.body." variable_sequence
request_path      ::= "request.path" | "request.path." variable_sequence
request_header    ::= "request.header." variable_sequence
variable_sequence ::= variable ('.' variable)*
select_index      ::= (sequence | variable)('[' rib-expr ']')+
sequence          ::= '[' rib-expr (',' rib-expr)* ']'
record            ::= '{' field (',' field)* '}'
tuple             ::= '(' rib-expr (',' rib-expr)* ')'
literal           ::= "'" [a-zA-Z0-9]* "'"
variable          ::= [a-zA-Z0-9_]+
number            ::= DIGITS
flags              ::= '{' STRING (',' STRING)* '}'
boolean           ::= "boolean" '(' BOOLEAN ')'
concat            ::= concat-elem*
concat-elem       ::= text | '${' variable '}'
text              ::= '[^${}]*'
multiple          ::= "multiple" '(' rib-expr* ')'
not               ::= "not" '(' rib-expr ')'
gt                ::= rib-expr '>' rib-expr
gt_or_eq_to       ::= rib-expr ">=" rib-expr
lt                ::= rib-expr '<' rib-expr
lt_or_eq_to       ::= rib-expr "<=" rib-expr
eq_to             ::= rib-expr "==" rib-expr
conditional       ::= "if" rib-expr "then" rib-expr "else" rib-expr
pattern_match     ::= "match" rib-expr '{' match_arms '}'
match_arms        ::= match_arm (',' match_arm)*
match_arm         ::= pattern '=>' rib-expr
pattern           ::= variable | ok | some | none | err | '_'
option            ::= "some" '(' rib-expr ')' | "none"
result            ::= "ok" '(' rib-expr ')' | "err" '(' rib-expr ')'
field_list         ::= field (',' field)*
field              ::= variable ':' rib-expr
pattern           ::= variable
                      | literal
                      | '_' // wildcard pattern

Here are some examples of each construct


Note that Rib expressions in the below examples need to be wrapped in code block using ${ }. This is to differentiate between raw text and an actual Rib program. As we progress, we may try to avoid the need of it, and being able to write Rib program in IDE without code-blocks. Example: ${1} is evaluated as a Number while 1 is evaluated as text.



This is a parsed as a number of type u64. Similarly -1 is parsed as a number of type i64 and 1.1 is parsed as a number of type f64.



This is parsed as a string literal. If the strings are not wrapped with quotes, then it is considered as a variable.



This is parsed as a boolean true. Similarly false is parsed as a boolean literal.



This is parsed as a Rib variable expression and evaluating such an expression can fail if the value of this variable is not available in the context of evaluation. Usually variables are used to refer to the values in the context of evaluation. The context can mostly be let binding or pattern matching or request or worker or response etc.

More explanations on variables can be found in other examples.


# Sequence of numbers
[1, 2, 3]
# Sequence of strings
['foo', 'bar', 'baz']
# Sequence of record
[{a: 'foo'}, {b : 'bar'}]

This is parsed as a sequence of values. While the values can be of any type, similar to any dynamic language, evaluation may fail with error if we mix different types in a sequence. In other words, Rib evaluator do expect the values in a sequence to be of the same type.


{ name: 'John', age: 30 }
{ city: 'New York', population: 8000000 }
{ country: 'France', capital: 'Paris' }
{ fruits: ['apple', 'banana', 'orange'], count: 3 }

This is parsed as a WASM Record. The syntax is inspired from WASM-WAVE. The minor difference here is strings are wrapped with single quotes instead of double quotes. Note that keys are not considered as variables. They are considered as literals (even in the absence of single quotes) if they are part of Record.


(1, 2, 3)

This is parsed as a tuple of values. Unlike sequence, the values in a tuple can be of different types.

('foo', 1, {a: 'bar'})


{ Foo, Bar, Baz }

This is parsed as Flag type in WASM. The values are separated by comma.

Selection of Field

A field can be selected from an Rib expression if it is a Record type. For example, foo.user is a valid selection given foo is a variable that gets evaluated to a record value.

{ name: 'John', age: 30 }.name

Selection of Index

This is selecting an index from a sequence value.

[1, 2, 3][0]

For example foo[10] is valid given foo is an Rib variable that gets evaluated to a sequence.

Request and selection of Fields

If using worker-bridge of golem OSS, you can use the variable request as this is available in the runtime context of Rib in worker-bridge that points to the input request.


To select the body field in request,


Say the request body is a record in Json, as given below. Rib sees it as a WASM Record type.

  "user": "Alice",
  "age": 30

Then we can use Rib language to select the field user, as it considers this request's body as a WASM Record type.


Worker Response and Selection of Fields

This is again available only if you use Rib along with worker-bridge of Golem. It points to the response from the worker. However, mostly you wouldn't need this as Rib allows you to call the worker's function and assign it to a variable.


One of the fields in worker Record is response. Therefore, to fetch the worker function's response, we can use the following expression which works in the same way as selecting a field in a record.


If the worker response is a record, then we can select the field in the response as follows

  "user": "Alice",
  "age": 30

Then we can use expr language to select the field user, as it considers the value of worker.response WASM Record type, and therefore a valid selection.


Resultin Rib is WASM Result, which can take the shape of ok or err. This is similar to Result type in std Rust.

A Result can be Ok of a value, or an Err of a value.


This is again, using the same syntax followed in wasm-wave. A worker's response can be at times a Result type, where it can either ok(x) or err(y) and if we use Rib language to peek/unwrap this result.

This can exist anywhere. If needed, user can embed a result in the request body and worker-bridge consider them to be a Result within the Record. More on these details can be found under worker-bridge's documentation

  "user": "ok(Alice)",
  "age": 30

Option Expr

Option in corresponding to WASM , which can take the shape of Some or None. This is similar to Option type in std Rust.

An Option can be Some of a value, or None.


The syntax is inspired from wasm wave.

Comparison Operators

5 > 3

The left side rib expression is evaluated as a Number 5, and the right side rib expression is evaluated as a Number 3. The comparison operator > is used to compare the two numbers, resulting in true.

Similarly, we can use other comparison operators like >=, <=, ==, < etc. Both operands should be a valid Rib code that points/evaluated to a number or string.

Trying to compare complex values will result in evaluation error. Example : [1, 2, 3] > [4, 5, 6] will result in error. We will add support for these in Expr language.

Conditional Statement

 if request.user.id > 3 then 'higher' else 'lower'

The structure of the conditional statement is if <condition> then <then-rib-expr> else <else-rib-expr>, where condition-expr is an expr that should get evaluated to boolean. The then-rib-expr or else-rib-expr can be an any valid rib code, which could be another if else itself

 if request.user.id > 3 then 'higher' else if request.user.id == 3 then 'equal' else 'lower'

If branch and then branch are not type checked as of now. Meaning similar to other dynamic languages like python, they may get evaluated to different types of WASM values.

Pattern Matching

match <expr> {
    <pattern> => <expr>,
    <pattern> => <expr>

Here are few examples that gives you an idea of what can be done with the current support of pattern matching. This would probably be your go-to construct when you are dealing with complex data structures like Result or Option or other custom variant (WASM Variant) that comes out as worker.response.

  match worker.response { ok(x) => 'foo', err(msg) => 'error' }

You can bring them in multiple lines as follows:

  match worker.response {
    ok(x) => 'foo',
    err(msg) => 'error'
  match worker.response {
    some(x) => 'found',
    none => 'not found'

Say your worker responded with a variant. Note on variant: A variant statement defines a new type where instances of the type match exactly one of the variants listed for the type. This is similar to a "sum" type in algebraic datatype (or an enum in Rust if you're familiar with it). Variants can be thought of as tagged unions as well.

Let's say we have a variant of type as given below responded from the worker. The worker-bridge is aware of the type of response from the worker.

variant my_variant {
    bar( {a: u32,b: string }),

Note that Rib doesn't allow you to define types as above. The above type is just there to show you the type of worker.response in this case.

  match worker.response {
    bar(x) => 'bar',
    foo(x) => 'foo'
Variables in the context of pattern matching

In all of the above, there exist a variable in the context of pattern. Example x in ok(x) or msg in err(msg) or x in some(x) or x in bar(x) or x in foo(x).

These variables are bound to the value that is being matched.

Example, given the worker response is ok(1), the following match expression will result in 2.

   let result = my-worker-function("foo");
   match result {
    ok(x) => x + 1,
    err(msg) => 0

The following match expression will result in 'c', if the worker response was variant value foo(['a', 'b', 'c']), and will result in 'a' if the worker.response was variant value bar({a: 1, b: 'a'}).

  let result = my-worker-function();
  match result {
    bar(x) => x.b,
    foo(x) => x[1]

More on let binding below

Wild card patterns

In some of the above examples, the binding variables are unused. In this case, we can use _ as a wildcard pattern to indicate that we don't care about the value.

  match worker.response {
    bar(_) => 'bar',
    foo(_) => 'foo'

Let Binding

 let a = { age: worker.response.age, city: 'New York', name: request.body.name };
 match worker.response.admin {
   Admin(_) => 'admin',
   User(x) => a

Or even as simple as:

  let x = { a : 1 };
  let y = { b : 2 };
  let z = x.a > y.b;