From 19252991a5d0acd2f7d1cd2b8f2294827c2f1edc Mon Sep 17 00:00:00 2001
From: Dmitry Boulytchev <dboulytchev@gmail.com>
Date: Thu, 24 Dec 2020 02:54:54 +0300
Subject: [PATCH] Added state monad

---
 src/version.ml  |  2 +-
 stdlib/Makefile |  2 +-
 stdlib/STM.lama | 45 +++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 47 insertions(+), 2 deletions(-)
 create mode 100644 stdlib/STM.lama

diff --git a/src/version.ml b/src/version.ml
index be85ec54e..ea69867c6 100644
--- a/src/version.ml
+++ b/src/version.ml
@@ -1 +1 @@
-let version = "Version 1.00, 66dc5c306, Tue Nov 24 23:11:17 2020 +0300"
+let version = "Version 1.00, 53b2efc3b, Fri Dec 11 01:22:25 2020 +0300"
diff --git a/stdlib/Makefile b/stdlib/Makefile
index e6ce887f0..3224cff78 100644
--- a/stdlib/Makefile
+++ b/stdlib/Makefile
@@ -18,7 +18,7 @@ Ostap.o: List.o Collection.o Ref.o Fun.o Matcher.o
 
 Buffer.o: List.o
 
-Expr.o: Ostap.o
+STM.o: List.o Fun.o
 
 %.o: %.lama
 	LAMA=../runtime $(LAMAC) -I . -c $<
diff --git a/stdlib/STM.lama b/stdlib/STM.lama
new file mode 100644
index 000000000..230e7d50f
--- /dev/null
+++ b/stdlib/STM.lama
@@ -0,0 +1,45 @@
+-- State Monad.
+-- (C) Dmitry Boulytchev, JetBrains Research, St. Petersburg State University, 2020
+--
+-- This unit provides a state
+
+import List;
+import Fun;
+
+public infixl => before $ (x, f) {
+  fun (state) {
+    case x (state) of
+      [state, x] -> [state, f (x)]
+    esac
+  }
+}
+
+public infix =>> at => (x, f) {
+  fun (state) {
+    case x (state) of
+      [state, x] -> f (x) (state)
+    esac
+  }
+}
+
+public fun returnST (x) {
+  fun (state) {[state, x]}
+}
+
+public fun chainST (xs) {
+  fun (state) {
+    case
+      foldl (fun (f, x) {
+               fun (state) {
+                 case f (state) of
+                   [state, xs] -> case x (state) of
+                                  [state, x] -> [state, x : xs]
+                                esac
+                 esac
+               }
+             }, returnST $ {}, xs) (state) of    
+      [state, xs] -> [state, reverse (xs)]
+    esac
+  }
+}
+ 
\ No newline at end of file