sema-eval 1.11.0

Trampoline-based evaluator and module system for the Sema programming language
Documentation 
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/// Built-in macros loaded at interpreter startup.
/// These expand to core special forms and don't require evaluator changes.
pub const PRELUDE: &str = r#"
;; Thread-first: inserts val as the FIRST argument of each form
;; (-> 5 (+ 3) (* 2)) => (* (+ 5 3) 2) => 16
(defmacro -> (val . forms)
  (if (null? forms)
    val
    (let ((form (car forms))
          (rest (cdr forms)))
      (if (list? form)
        `(-> (,(car form) ,val ,@(cdr form)) ,@rest)
        `(-> (,form ,val) ,@rest)))))

;; Thread-last: inserts val as the LAST argument of each form
;; (->> (range 10) (filter odd?) (map square)) => (map square (filter odd? (range 10)))
(defmacro ->> (val . forms)
  (if (null? forms)
    val
    (let ((form (car forms))
          (rest (cdr forms)))
      (if (list? form)
        `(->> (,(car form) ,@(cdr form) ,val) ,@rest)
        `(->> (,form ,val) ,@rest)))))

;; Thread-as: binds val to a name, allowing arbitrary placement
;; (as-> 5 x (+ x 3) (* x x) (- x 1)) => 63
(defmacro as-> (val name . forms)
  (if (null? forms)
    val
    (let ((form (car forms))
          (rest (cdr forms)))
      `(let ((,name ,val))
         (as-> ,form ,name ,@rest)))))

;; Conditional thread-first: short-circuits on nil
;; (some-> m :key :nested) => nil if any step returns nil
(defmacro some-> (val . forms)
  (if (null? forms)
    val
    (let ((form (car forms))
          (rest (cdr forms)))
      (if (list? form)
        `(let ((v# ,val))
           (if (nil? v#) nil (some-> (,(car form) v# ,@(cdr form)) ,@rest)))
        `(let ((v# ,val))
           (if (nil? v#) nil (some-> (,form v#) ,@rest)))))))

;; when-let: bind a value, execute body only if non-nil
;; (when-let (x (get m :key)) (println x))
(defmacro when-let (binding . body)
  (let ((var (car binding))
        (expr (cadr binding)))
    `(let ((,var ,expr))
       (when (not (nil? ,var))
         ,@body))))

;; if-let: bind a value, branch on nil/non-nil
;; (if-let (x (get m :key)) (use x) (default))
(defmacro if-let (binding then else)
  (let ((var (car binding))
        (expr (cadr binding)))
    `(let ((,var ,expr))
       (if (nil? ,var) ,else ,then))))
"#;