eval

type Boolean struct {
	Val bool `json:"result"`
}

type Env map[Var]Substance

type Expr interface {
	// Eval returns the value of this Expr in the environment env.
	Eval(env Env) Substance
	// Check reports errors in this Expr and adds its Vars to the set.
	Check(vars map[Var]bool) error
}

type Float64 struct {
	Val float64 `json:"result"`
}

type Formula func(args []Expr, env Env) Substance

type FormulaReq struct {
	Expression string                 `json:"expression"`
	Parameter  map[string]interface{} `json:"parameter"`
}

type Result interface{}

type String struct {
	Val string `json:"result"`
}

type Substance interface {
	GetType() T
	String() string
	Float64() float64
	Boolean() bool
}

type T string

type Var string