equation

struct Equation
Equation(lhs::ASTExpr, rhs::ASTExpr; _comment)

Equation(lhs::ASTExpr, rhs::ASTExpr; _comment=nothing)

class Equation
Equation(lhs: Expr, rhs: Expr, _comment: str | None = None)

# Edits return a new model in `modified_object`; the input is left unchanged.
edited = earthsci_ast.add_variable(
    model, "N0", ModelVariable(type="parameter", units="mol", default=100.0)
).modified_object
edited = earthsci_ast.add_variable(
    edited, "fraction", ModelVariable(type="unknown", units="1")
).modified_object
step = earthsci_ast.add_equation(
    edited, Equation(lhs="fraction", rhs=ExprNode(op="/", args=["N", "N0"]))
)
assert step.success, step.errors
doc.models["Decay"] = step.modified_object
assert "fraction" not in model.variables  # the original model is untouched

assert earthsci_ast.validate(doc).is_valid
assert earthsci_ast.observed_unknowns(doc.models["Decay"]) == ["fraction", "half_life"]

text = earthsci_ast.to_json(doc)
assert json.loads(text)["models"]["Decay"]["equations"][-1] == {
    "lhs": "fraction",
    "rhs": {"op": "/", "args": ["N", "N0"]},
}

with tempfile.TemporaryDirectory() as tmp:
    out = Path(tmp) / "decay_edited.esm"
    earthsci_ast.write_path(doc, out)
    reread = earthsci_ast.load_path(out)
assert list(reread.models["Decay"].variables) == ["N", "lambda", "half_life", "N0", "fraction"]

pub struct Equation {
    pub lhs: Expr,
    pub rhs: Expr,
    pub comment: Option<String>,
}

type Equation struct {
	LHS Expression `json:"lhs"`
	RHS Expression `json:"rhs"`
	// Comment is the schema-sanctioned `_comment` annotation (esm-schema.json
	// `$defs/Equation`): free prose the author attached to THIS equation. The
	// core carries no meaning for it, but it is authored content, so dropping
	// it on parse → emit silently strips every equation annotation from a
	// document edited through this binding. `omitempty`: the key is optional
	// and absent from the overwhelming majority of equations, so emitting a
	// null (or an empty string) for the unannotated case would add noise to
	// every document — and `"_comment": null` fails the schema's `string` type.
	//
	// Rebuilds that construct a NEW Equation from an old one (substitution,
	// namespacing, flattening) must carry it across explicitly; a fresh
	// Equation{LHS, RHS} literal would drop it.
	Comment *string `json:"_comment,omitempty"`
}

func (e *Equation) UnmarshalJSON(data []byte) error

func (e *Equation) UnmarshalJSON(data []byte) error