; Fieldtest mut-local #2 — classify a temperature into a band using
; nested if-branches that each update a mut-Int "category code".
;
; Task: given a temperature value, set a category-code mut-var to
; 0 (freezing), 1 (cold), 2 (warm), 3 (hot) by walking through a
; cascade of if-branches. Print the resulting code.
;
; Why this fits mut-local's scope: this exercises mut composed with
; `if` — each branch contains a single `(assign ...)`. The seal-by-
; construction promise says the if-branch can write to the var, and
; the var's value flows out of the branch as the latest store. This is
; a use of mut that *replaces* what a chain of let-rebinds would
; otherwise do, and a chain of let-rebinds is the AILang author's
; usual workaround for "set this variable conditionally" — so the
; mut form should be measurably cleaner here.
;
; Expected stdout: 2  (room temperature 22 = "warm")

(module mut-local_2_classify_temp

  (fn classify
    (doc "Return category code 0..3 for temperature t in degrees C.")
    (type (fn-type (params (con Int)) (ret (con Int))))
    (params t)
    (body
      (mut
        (var code (con Int) 0)
        (if (app < t 0)
          (assign code 0)
          (if (app < t 15)
            (assign code 1)
            (if (app < t 28)
              (assign code 2)
              (assign code 3))))
        code)))

  (fn main
    (type (fn-type (params) (ret (con Unit)) (effects IO)))
    (params)
    (body
      (app print (app classify 22)))))