# std/contract

Check a module against a module contract.

`checkContract` compares a `contract` declaration with a module's `moduleInterface` and returns `Issue`s:

```vyrn
import { checkContract } from "std/contract"

```

```vyrn
gen fn pages(dir: String) -> String {
    let iface = moduleInterface(pageFile)
    let issues = checkContract(iface, contractOf(Page))
    ...
}

```

The compiler knows only the declaration form, so a third-party generator can declare its own contract. Every condition is reported:

- required member absent            -> `contract.missing`
- member type mismatch              -> `contract.type`
- unknown export, close to a member -> `contract.unknown.didYouMean`
- unknown export, not close         -> `contract.unknown`
- open-rule shape mismatch          -> `contract.open`

A module exports only functions (every top-level `let` is private), so a `let` member is satisfied by an accessor (`export fn head() -> Head`). A member with a default (`fn head() -> Head = noHead()`) is optional. A `fn` member name declared more than once has alternative signatures; the module matches any one, the name is optional when any alternative has a default, and `matchedMember` says which one matched:

```vyrn
fn head() -> Head = noHead()
fn head(d: T) -> Head
fn head(p: P) -> Head
fn head(p: P, d: T) -> Head

```

## checkContract

`fn`, at line 366.

```vyrn
fn checkContract(iface: ModuleInterface, c: ContractInfo) -> Array<Issue>
```

One `Issue` per problem, in a fixed order: the contract's members in declaration order, then the module's unrecognized exports in reflection order. Empty when the module satisfies the contract.

## suppliesMember

`fn`, at line 434.

```vyrn
fn suppliesMember(iface: ModuleInterface, c: ContractInfo, name: String) -> Bool
```

Whether `iface` exports member `name` at one of its declared shapes.

## matchedMember

`fn`, at line 441.

```vyrn
fn matchedMember(iface: ModuleInterface, c: ContractInfo, name: String) -> Int64
```

The 0-based index of the alternative of member `name` the module supplies, in declaration order, or `0 - 1` when it supplies none. Compares in place, with no `Export` list built.

---

From https://vyrn-lang.github.io/docs/std/contract.html — generated by `site/export.vyrn` from the Vyrn repository.