5 files changed, 48 insertions, 0 deletions
diff --git a/algebraic-structures.egg b/algebraic-structures.egg
index 9b21678..bf62bd3 100644
--- a/algebraic-structures.egg
+++ b/algebraic-structures.egg
@@ -16,6 +16,9 @@
   (extension algebraic-structures.monoid.number.product.base)
   (extension algebraic-structures.monoid.number.product)
   (extension algebraic-structures.monoid.make.fold)
+  (extension algebraic-structures.group.make)
+  (extension algebraic-structures.group.number.sum)
+  (extension algebraic-structures.group.number.product)
   (extension algebraic-structures.foldable.make)
   (extension algebraic-structures.foldable.list.base)
   (extension algebraic-structures.foldable.list)
diff --git a/algebraic-structures.group.make.scm b/algebraic-structures.group.make.scm
new file mode 100644
index 0000000..7568ac9
--- /dev/null
+++ b/algebraic-structures.group.make.scm
@@ -0,0 +1,16 @@
+(functor ((algebraic-structures group make) (M (<> unit inv)))
+    (<> unit inv pow)
+  (import M
+          scheme
+          (chicken base))
+
+  (define (pow x n)
+    (assert (exact-integer? n))
+    (if (negative? n)
+        (pow (inv x) (- n))
+        (let loop ((i n)
+                   (acc unit))
+          (if (= i 0)
+              acc
+              (loop (sub1 i)
+                    (<> acc x)))))))
diff --git a/algebraic-structures.group.number.product.scm b/algebraic-structures.group.number.product.scm
new file mode 100644
index 0000000..a57cdf6
--- /dev/null
+++ b/algebraic-structures.group.number.product.scm
@@ -0,0 +1,8 @@
+(import (only (algebraic-structures group make)))
+(module (algebraic-structures group number product) = (algebraic-structures group make)
+  (import scheme
+          (chicken module)
+          (algebraic-structures monoid number product))
+  (export <> unit inv)
+
+  (define (inv x) (/ x)))
diff --git a/algebraic-structures.group.number.sum.scm b/algebraic-structures.group.number.sum.scm
new file mode 100644
index 0000000..2bb7331
--- /dev/null
+++ b/algebraic-structures.group.number.sum.scm
@@ -0,0 +1,8 @@
+(import (only (algebraic-structures group make)))
+(module (algebraic-structures group number sum) = (algebraic-structures group make)
+  (import scheme
+          (chicken module)
+          (algebraic-structures monoid number sum))
+  (export <> unit inv)
+
+  (define (inv x) (- x)))
diff --git a/tests/run.scm b/tests/run.scm
index ed2cd8f..6b6a74e 100644
--- a/tests/run.scm
+++ b/tests/run.scm
@@ -107,6 +107,19 @@
 
 (test-end "monoid")
 
+(test-begin "group")
+
+(import (prefix (algebraic-structures group number product) product:))
+(import (prefix (algebraic-structures group number sum) sum:))
+
+(test -9 (sum:inv 9))
+(test 9 (sum:pow 3 3))
+
+(test 1/9 (product:inv 9))
+(test 9 (product:pow 3 2))
+
+(test-end "group")
+
 (test-begin "functor")
 
 (test '((a) (b) (c)) (list:map list '(a b c)))