calchylus.hy

; index-find function needs to be available at compile time for the lambda expression macro
(eval-and-compile
  ; set comma constant for separator
  ; at the moment Hy doesn't support dot on macro expressions because dot is mixed with internal HyCons functionality
  ; causing this error: https://github.com/hylang/hy/blob/e8ffd412028232cc2cc4fe4bfb10f21ce8ab2565/hy/compiler.py#L2478
  (setv comma '·)
  ; find index of the element from the list. if the element is not found, return -1
  (defn index-find [elm lst]
    (try (.index lst elm) (except [ValueError] -1))))
; lambda expression macro
; for example: (𝜆 x y · x 1 0) ; -> 1
(defmacro 𝜆 [&rest expr]
  (setv idx (index-find comma expr))
  `(fn ~(cut expr 0 (if (pos? idx) idx 0)) ~@(cut expr (inc idx))))