# fml-check
Type checker for the [FHIR Mapping Language](https://build.fhir.org/ig/HL7/mapping-language-ig/):
finds what a map gets wrong before it runs.
It follows every rule with the types of its variables and reports:
- elements that do not exist on the type they are read from or written to, including inherited
ones, backbone elements and content references (`Questionnaire.item.item`);
- type annotations that are not a type of the element (`src.deceased : string`);
- unknown variables and writes into source variables;
- unknown or abstract types in `create('Type')`, and transforms with the wrong number of arguments;
- calls to groups that are not declared, or with the wrong number of arguments;
- element paths inside FHIRPath (`where`, `check`, `(…)`), as warnings, since a wrong path there
gives an empty result instead of an error;
- what the Mapping Language IG requires of a map: a `url` and a `name`, groups that extend another
with its parameters (mode, name and type), unique constant names; and, as warnings because the
HL7 maps themselves break them, default groups that are not a typed source and target, rule
names used twice, and reserved keywords used as names.
Groups without parameter types are checked with the types they are called with, which is how the
cross-version maps are written. Those maps (R2–R5) check without errors; on other public maps it found a
typo in the FHIR test cases, a parameter a group uses without declaring it, and a condition that is
never true because it reads `system` and `code` from a CodeableConcept instead of its codings.
Definitions come through a small `Definitions` trait. With the `fhirschema` feature,
`CoreDefinitions` provides FHIR R4, R4B, R5 and R6 (6.0.0-ballot5), including the
version-specific URLs the cross-version maps use (`http://hl7.org/fhir/4.0/StructureDefinition/…`).
`PackageDefinitions` puts the StructureDefinitions of FHIR packages (profiles, extensions, logical
models) in front of them.
```rust
use fml_check::{CoreDefinitions, Release, check};
let parse = fml_syntax::parse(source);
for diagnostic in check(&parse.tree(), &CoreDefinitions::new(Release::R5)) {
eprintln!("{:?}: {}", diagnostic.severity, diagnostic.message);
}
```
Part of [fml-rs](https://github.com/octofhir/fml-rs). Licensed under MIT or Apache-2.0.