Contracts in Nickel

Note: Most of the examples of this document are run in the Nickel REPL. To start a session, run nickel repl in your terminal.

For the motivation behind contracts and a high-level overview of contracts and types, first read the correctness section.

To a first approximation, contracts are assertions. They check that a value satisfies some property at run-time. If the test passes, the execution can go on normally. Otherwise, an error is raised. In Nickel, you can apply a contract using the | operator:

let x = (1 + 1 | Number) in x

Contracts can also be attached to identifiers in a let binding. The following is equivalent to the previous example:

let x | Number = 1 + 1 in x

Contracts can also be attached to record fields:

{x | Number = 1 + 1}

Important: contracts attached to a record field, such as {x | Number = 1}, and a freestanding contract annotation such as {x = (1 | Number)} behave differently with respect to record merging. However, as far as runtime checks are concerned (which is the focus of this document), they are equivalent.

Here, x is bound to a Number contract. When evaluating x, the following steps are performed:

  1. Evaluate 1 + 1.
  2. Check that the result is a number.
  3. If it is, return the expression unchanged. Otherwise, raise an error.
> 1 + 1 | Number
2

> "a" | Number
error: contract broken by a value
[...]

Contracts corresponding to the basic types Number, String, Bool and Dyn are available. Dyn is a contract that never fails.

User-defined contracts

Preamble: idempotency

As you will see in this section, Nickel contracts are more than mere boolean validators. They can sometimes modify the checked value for valid reasons exposed below. However, it's hard (if not impossible) to rightfully limit the expressive power of contracts to be reasonable with respect to modifications.

For Nickel, the notion of reasonable modification is practically defined by idempotency. An idempotent contract is a contract which gives the same result if applied once or many times to the same value. That is, for any value, MyContract is idempotent if value | MyContract and value | MyContract | MyContract gives the same result. Idempotency sounds like a reasonable property for a contract, even if it can perform some normalization.

Nickel makes the assumption that all contracts - including user-defined contracts - are idempotent, because the converse would be surprising for consumers of the contract and because this assumption enables important optimizations such as contract deduplication. While non-idempotent contracts shouldn't wreak havoc on your program, they might lead to surprising results, such as a change in behavior e.g. when refactoring a program to a new version that should be identical, typically because the deduplication optimization doesn't fire anymore.

By hand

The ability to check arbitrary properties is where run-time contracts really shine. Let us see how to define our very own contract. Here is an example of a custom contract:

{
  IsFoo = fun label value =>
    if std.is_string value then
      if value == "foo" then
        value
      else
        std.contract.blame_with_message "not equal to \"foo\"" label
    else
      std.contract.blame_with_message "not a string" label,
}

A custom contract is a function of two arguments:

  • A label. The label is provided by the interpreter and contains tracking information for error reporting. The label can be used by the contract to customize the error reporting as well.
  • The value being checked.

Upon success, the contract must return the original value. We will see the reason why in the laziness section. To signal failure, a custom contract uses std.contract.blame. Custom contracts can use the label to customize error reporting upon failure using the functions from std.contract.label, which set various attributes of the label. std.contract.blame_with_message message label is just shorthand for:

label
|> std.contract.label.with_message message
|> std.contract.blame

blame immediately aborts the execution and reports a contract violation error.

In IsFoo, we first test if the value is a string, and then if it is equal to "foo", in which case the contract succeeds. Otherwise, the contract fails with appropriate error messages. Let us try:

> 1 | IsFoo
error: contract broken by a value: not a string
[...]

> "a" | IsFoo
error: contract broken by a value: not equal to "foo"
[...]

> "foo" | IsFoo
"foo"

With from_predicate

Our contract is simple: in the end, it tests the condition value == "foo". Unfortunately, it has a few cascading ifs that don't look very nice. This is a necessary evil if you want to provide distinct error messages (not a string and not a "foo"). However, one could argue in this case that the information of the contract's name (which is printed for contract violation errors amongst other data) is enough to understand the error. In this case, we can write our contract more succinctly as:

{ IsFoo = std.contract.from_predicate ((==) "foo") }

std.contract.from_predicate takes a predicate (a function of one argument which returns a boolean) and converts it to a contract. The syntax (==) turns the equality operator == into a function, and is shorthand for fun x y => x == y. The partial application (==) "foo" is then the function fun y => "foo" == y, which is exactly the condition we want. from_predicate is useful to quickly define contracts based on a boolean condition, and when the contract is simple enough to not require a custom error message.

Here is an example of a port number contract:

{
  Port =
    std.contract.from_predicate
      (
        fun value =>
          std.is_number value
          && std.number.is_integer value
          && value >= 0
          && value <= 65535
      )
}

Parametrized contracts

Let us consider a contract for bounds checking:

let Between5And10 =
  std.contract.from_predicate
    (
      fun value =>
        std.is_number value
        && value >= 5
        && value <= 10
    )
in
let Schema = {
  level | Between5And10,
}
in

{ level = 5 } | Schema

Now, we add a new field to our schema, which must be between 0 and 1:

let Between5And10 =
  std.contract.from_predicate
    (
      fun value =>
        std.is_number value
        && value >= 5
        && value <= 10
    )
in
let Between0And1 =
  std.contract.from_predicate
    (
      fun value =>
        std.is_number value
        && value >= 0
        && value <= 1
    )
in
let Schema = {
  level | Between5And10,
  strength | Between0And1,
}
in

{
  level = 5,
  strength = 0.5,
} | Schema

Between5And9 and Between0And1 are awkwardly similar. Because custom contracts are nothing more than functions, we can use function arguments as parameters for contracts. Given how partial function application works, parameters must appear first, before the label and value arguments:

let Between = fun min max =>
  std.contract.from_predicate (fun value =>
    value >= min &&
    value <= max) in
let Schema = {
  level | Between 5 10,
  strength | Between 0 1,
}
in

{
  level = 5,
  strength = 0.5,
} | Schema

Contracts parametrized by contracts

Contracts parametrized by other contracts are not really special amongst parametrized contracts, but note that although contracts can be functions, we will see soon that they can be different objects as well. Even with a contract defined as a function, contract application behaves differently than bare function application. Thus, when manually handling another contract Contract, do not apply it as a function Contract label value, but use std.contract.apply Contract label value instead.

One example of a contract parametrized by another contract is a Nullable contract, that accepts a value that is either null or of some other given format:

let Nullable = fun contract label value =>
  if value == null then
    value
  else
    std.contract.apply contract label value
in
[
  # succeeds
  null | Nullable Number,
  # succeeds
  1 | Nullable Number,
  # fails
  "a" | Nullable Number,
]

Compound contracts

This section describes the different ways of composing existing contracts together to create new ones.

Records

Record contracts are fundamental in a configuration language like Nickel. They encode the data schema of a configuration. A record contract is a record literal with contract annotations and whose field definitions are usually (but not necessarily) missing:

let Schema = {
  path | String,

  connection
    | {
      server_port | Port,
      host | String,
    }
}
in

{
  path = "/foo/bar",
  connection = {
    server_port =
      if host == "localhost" then
        "8080"
      else
        80,
    host = "localhost",
  }
} | Schema

Schema is a schema describing a configuration with:

  • a field path that must be a string
  • a sub-record connection, which has a server_port and a host field that are respectively a port number and a string.

See how we've combined contracts: while defining the Schema record contract, we wrote another inline record contract for the connection field, and we reused the Port contract defined earlier. Composition is this ability of using existing contracts seamlessly as building blocks for new contracts.

If we export this example to JSON, we get:

$ nickel -f config.ncl export
error: contract broken by the value of `server_port`
   ┌─ example.ncl:26:7
   │
16 │         server_port | Port,
   │                       ---- expected type
   ·
26 │ ╭       if host == "localhost" then
27 │ │         "8080"
   │ │         ------ evaluated to this expression
28 │ │       else
29 │ │         80,
   │ ╰──────────^ applied to this expression
   │
   ┌─ <unknown> (generated by evaluation):1:1
   │
 1 │ "8080"
   │ ------ evaluated to this value

Indeed, our server_port is a string, while a number was expected. If we replace "8080" by 8080, we finally obtain:

{
  "connection": {
    "host": "localhost",
    "server_port": 8080
  },
  "path": "/foo/bar"
}

If for a specific use-case port numbers can actually also be specified as strings, you can amend the Port contract to be more permissive and accept strings representing valid port numbers.

Metadata

In addition to defining the contracts of fields, record contracts can also attach metadata (see merging) to fields, such as documentation or default values:

> let Schema = {
    foo
      | doc "This documentation will propagate to the final value!"
      | String
      | default
      = "foo",
    bar | Number,
  }

> let config | Schema = {bar = 2}

> std.serialize 'Json config
"{\n  \"bar\": 2,\n  \"foo\": \"foo\"\n}"

> :query config foo
* contract: String
* default: "foo"
* documentation: This documentation will propagate to the final value!

Open record contracts

By default, record contracts are closed, meaning that additional fields are forbidden:

> let Contract = {foo | String}

> {foo = "a", bar = 1} | Contract
error: contract broken by a value: extra field `bar`
[...]

If you want to allow additional fields, append , .. after the last field definition to define an open contract:

> let Contract = {foo | String, ..}

> {foo = "a", bar = 1} | Contract
{ bar = 1, foo | String = "a", .. }

Giving values to fields

While most record contracts don't have field definitions, they can. In fact, record contracts are nothing special: any record literal can be used both as a contract or a normal value. If a field is defined both in the contract and the checked value, the two definitions will be merged in the final result. For example:

> let Secure = {
    must_be_very_secure | Bool = true,
    data | String,
  }

> std.serialize 'Json ({data = ""} | Secure)
"{\n  \"data\": \"\",\n  \"must_be_very_secure\": true\n}"

> {data = "", must_be_very_secure = false} | Secure
error: non mergeable terms
  ┌─ <repl-input-15>:1:361{data = "", must_be_very_secure = false} | Secure
  │                                    ^^^^^    ------ originally merged here
  │                                    │
  │                                    cannot merge this expression
  │
  ┌─ <repl-input-13>:2:342 │     must_be_very_secure | Bool = true,
  │                                  ^^^^ with this expression
  │
  = Both values have the same merge priority but they can't be combined.
  = Primitive values (Number, String, and Bool) or arrays can be merged only if they are equal.
  = Functions can never be merged.

Warning: = vs |

It may be tempting to use = instead of | to attach a record contract to a field. That is, writing Contract = {foo = {bar | String}} instead of Contract = {foo | {bar | String}}. When applying this contract, the merging operator will apply the String contract to the field foo of the checked value. At first sight, = also fits the bill. However, there are a number of subtle but potentially surprising differences.

One concerns open contracts. Merging never requires the presence of specific fields: thus, the contract {bar | String} attached to foo will actually behave like an open contract, even if you didn't use ... This might or might not be what you want:

> let ContractPipe = {
    sub_field | {foo | String}
  }

> let ContractEq = {
    sub_field = {foo | String}
  }

> {sub_field.foo = "a", sub_field.bar = "b"} | ContractPipe
error: contract broken by the value of `sub_field`: extra field `bar`
[...]

> {sub_field.foo = "a", sub_field.bar = "b"} | ContractEq
{ sub_field = { bar = "b", foo | String = "a", }, }

There are other discrepancies, e.g. when applying the contract to a record with a default annotation on subfield. Thus, unless you have a specific use-case in mind, you should use | instead of = when attaching record contracts.

Type constructors for contracts

We've already seen that the primitive types Number, String and Bool can be used as contracts. In fact, any type constructor of the static type system can be used to combine contracts.

Array

An array contract checks that the value is an array and applies its parameter contract to each element:

> let VeryBig =
  std.contract.from_predicate (
      fun value =>
        std.is_number value
        && value >= 1000
    )

> [1000, 10001, 2] | Array VeryBig
error: contract broken by a value
  ┌─ <repl-input-21>:1:161 │  [1000, 10001, 2] | Array VeryBig
  │                ^          ------- expected array element type
  │                │
  │                applied to this expression

Functions

A function contract In -> Out returns a wrapped function which, for each call, will check the parameter against the In contract and the return value against the Out contract. Put differently, In represents pre-conditions which must hold for the parameter, and Out represents post-conditions which must hold for the return value of the function.

Caller vs callee

Function contracts, as opposed to a contract like Number, have the peculiarity of involving two parties. For example:

let add_semi | String -> String = fun x => x ++ ";" in
add_semi 1

let wrong | String -> String = fun x => 0 in
wrong "a"

Both of those examples will fail with a contract violation. But they are different in nature: in the first one, the function add_semi respects its contract. Whenever x is a string, add_semi does return a string. Here, the caller (the user of the function) is to blame, and not the callee (the function). This is an important distinction. Say you wrote add_semi as part of a library. A wrong call is a random user of your library, somewhere, misusing add_semi. This is not your responsibility to fix, assuming the contract error messages are good enough to let users understand the issue quickly.

The second example is the converse. The caller provides a string, as requested by the contract, but the function returns a number. The blame is on the callee. If you are the library writer who shipped the wrong function, this is a bug you have to fix.

The interpreter automatically performs bookkeeping for function contracts in order to make this caller/callee distinction:

> let add_semi | String -> String = fun x => x ++ ";" in
  add_semi 1
error: contract broken by the caller
[...]

> let wrong | String -> String = fun x => 0 in
  wrong "a"
error: contract broken by a function
[...]
Higher-order functions

The beauty of function contracts is that they gracefully scale to higher-order functions as well. Higher-order functions are functions that take other functions as parameters. Here is an example:

> let apply_fun | (Number -> Number) -> Number = fun f => f 0 in
  apply_fun (fun x => "a")
error: contract broken by the caller
  ┌─ <repl-input-24>:1:291let apply_fun | (Number -> Number) -> Number = fun f => f 0 in------ expected return type of a function provided by the caller
2 │   apply_fun (fun x => "a")--- evaluated to this expression
  │
  ┌─ <unknown> (generated by evaluation):1:11"a"--- evaluated to this value
[...]

Dictionary

The type constructor {_ : Contract} represents a record whose field names are not constrained but whose field values must satisfy Contract. In practice, Contract is applied to each field. Such a contract is useful when using records as an extensible dictionary, that is a key/value store, where keys are strings and values satisfy Contract, for example:

> let occurrences | {_: Number} = {a = 2, b = 3, "!" = 5, "^" = 1} in
  occurrences."!"
5

Laziness

In the section on writing a custom contract by hand, we noted the strange fact that a custom contract must return a value, instead of just returning e.g. a boolean to indicate success or failure. A contract could even always return null, as failure is handled separately by aborting. Moreover, the contracts we have written so far always returned the original unmodified value upon success, which doesn't sound very useful: after all, the caller and the interpreter already had access to this value to begin with.

The motivation for this return value is laziness. Nickel is designed to be lazy, only evaluating values on-demand.

For example, record fields are not evaluated until they are accessed, printed (in the REPL or as a result of nickel), or serialized/exported. Thanks to laziness, you can for example query specific information on a large configuration without having to actually evaluate everything. We will use the always failing contract std.FailWith to observe where evaluation takes place:

> let config = {
    fail | std.FailWith "ooch" = null,
    data | doc "Some information" = 42
  }

> :query config data
* documentation: Some information

> config.fail
error: contract broken by the value of `fail`: ooch
[...]

See how the command :query config.data succeeds, although the field fail causes an error when evaluated.

A large configuration will most probably have a root contract attached, corresponding to its schema. If this contract checked everything eagerly, forcing the evaluation of most of the configuration, we would lose the benefits of laziness. Thus, we want contracts to be lazy as well. In particular, a subcontract attached to a field should only fire when the field is requested. This is the case with record contracts, and in general with all native contracts combinators.

Writing lazy contracts

Imagine we want to write a contract similar to {_ : Bool}, that is a dictionary of booleans, but we also want keys to be number literals (although represented as strings). A valid value could look like {"1": true, "2": false, "10": true}. If we used boolean predicates as the default for contracts, it would be impossible to make it lazy: as soon as your contract is called, you would need to produce a true or false answer, and checking that fields are all Bool requires evaluating them first.

What we can do is to not perform all the checks right away, but return a new value which is wrapping the original value with delayed checks inside. This is the rationale behind contracts returning a value. Let us see:

{
  NumberBoolDict = fun label value =>
    if std.is_record value then
      let check_fields =
        value
        |> std.record.fields
        |> std.array.fold_left
          (
            fun acc field_name =>
              if std.string.is_match "^\\d+$" field_name then
                acc # unused and always null through iteration
              else
                std.contract.blame_with_message "field name `%{field_name}` is not a number" label
          )
          null
      in
      value
      |> std.record.map
        (
          fun name value =>
            let label_with_msg =
              std.contract.label.with_message "field `%{name}` is not a boolean" label
            in
            std.contract.apply Bool label_with_msg value
        )
      |> std.seq check_fields
    else
      std.contract.blame_with_message "not a record" label
}

There is a lot to unwrap here. Please refer to the syntax section if you have trouble parsing the example. We first check that the value is a record on line 3. We then define check_fields on line 4, an expression that goes over all the record field names and check that each one is a sequence of digits. We use a left fold with a dummy null accumulator as a way to just iterate through the array without building up anything interesting.

For laziness, the interesting bit happens here:

value
|> std.record.map
  (
    fun name value =>
      let label_with_msg =
        std.contract.label.with_message "field `%{name}` is not a boolean" label
      in
      std.contract.apply Bool label_with_msg value
  )
|> std.seq check_fields

This is the final return value of the contract (at least when value is a record). This code takes the original record, and maps a function over it which substitutes each field for the same value but wrapped in a Bool contract. Because records (and record mapping) are lazy, this doesn't actually execute the Bool contracts right away. Each contract will only be run when the corresponding field will be requested.

Finally, we sequence the result with check_fields: if we remove this last line and return the mapped value directly, because Nickel is lazy and check_fields is not used elsewhere, the check wouldn't actually trigger. Using seq first forces the evaluation of check_fields unconditionally, ignores the resulting value and continues with the second argument (here, our wrapped value).

Let us see if we indeed preserved laziness:

> let config | NumberBoolDict = {
    "1" | std.FailWith "ooch" = null, # same as our previous "fail"
    "0" | doc "Some information" = true,
  }

> config."0"
true

Yes! Our contract doesn't unduly cause the evaluation of the field "1". Does it check anything, though?

> let config | NumberBoolDict = {
    not_a_number = false,
    "0" | doc "Some information" = false,
  }

> config."0"
error: contract broken by a value: field name `not_a_number` is not a number
[...]

> let config | NumberBoolDict = {
    "0" | doc "Some information" = "not a boolean",
  }

> config."0"
error: contract broken by a value: field `0` is not a boolean
[...]

It does!

Remark: lazy, always?

Note that our check that field names are digits is not lazy in the sense that even when requesting another field "0", we've already triggered this check for all field names. When evaluating a record lazily, Nickel must still determine its structure and in particular the field names. What's not evaluated right away is the content of each field.

Thus, performing the check for field names right away is fine: early error reporting is a good thing, and the field names are readily available, so we don't force the evaluation of something that wouldn't be evaluated without the contract. We could have made this check lazy by putting it inside each field, together with the Bool contract application.

Conclusion

Our NumberBoolDict contract doesn't perform all the checks needed right away. Instead, it returns a new value, which is wrapping the original value with delayed checks inside. Doing so preserves laziness of the language and only triggers the checks when the values are used or exported in a configuration. This is the reason for contracts to return a value, which must be the original value with potential delayed checks inside.