From b8b9fee9ceaf7ec0cd1b40f7908734cca89a3200 Mon Sep 17 00:00:00 2001
From: Sergiu Ivanov <sivanov@colimite.fr>
Date: Wed, 29 Mar 2023 17:46:53 +0200
Subject: [PATCH] Type sbf/state?.

---
 scribblings/tbn.scrbl | 10 ++++++++++
 tbn.rkt               |  9 +++++++++
 2 files changed, 19 insertions(+)

diff --git a/scribblings/tbn.scrbl b/scribblings/tbn.scrbl
index 044e0da..88780be 100644
--- a/scribblings/tbn.scrbl
+++ b/scribblings/tbn.scrbl
@@ -81,3 +81,13 @@ and weights, as well as a threshold.
 @ex[
 (make-tbf/state '((x1 . 1) (x2 . 1)) 1)
 ]}
+
+@defproc[(sbf/state? [tbfs TBF/State]) Boolean]{
+
+A state sign Boolean function (SBF) is a @racket[TBF/State] whose
+threshold is 0.
+
+@ex[
+(sbf/state? (tbf/state (hash 'a -1 'b 1) 0))
+(sbf/state? (tbf/state (hash 'a -1 'b 1) 1))
+]}
diff --git a/tbn.rkt b/tbn.rkt
index 116f805..4973b12 100644
--- a/tbn.rkt
+++ b/tbn.rkt
@@ -16,6 +16,7 @@
    apply-tbf-to-state
 
    (struct-out tbf/state) TBF/State tbf/state-w tbf/state-θ make-tbf/state
+   sbf/state?
    )
 
   (: apply-tbf-to-state (-> TBF (State (U Zero One)) (U Zero One)))
@@ -45,6 +46,14 @@
       (define f (make-tbf/state '((x1 . 1) (x2 . 1)) 1))
       (check-equal? (tbf/state-w f) #hash((x1 . 1) (x2 . 1)))
       (check-equal? (tbf/state-θ f) 1)))
+
+  (: sbf/state? (-> TBF/State Boolean))
+  (define (sbf/state? tbfs) (zero? (tbf/state-θ tbfs)))
+
+  (module+ test
+    (test-case "sbf/state?"
+      (check-true (sbf/state? (tbf/state (hash 'a -1 'b 1) 0)))
+      (check-false (sbf/state? (tbf/state (hash 'a -1 'b 1) 1)))))
   )
 
 (module+ test