6.1.2. Write Your First Unit Test

Fix an existing unit test

The example business logic allows account balances to go negative. Let's enhance this logic by adding a constraint that account balances can never go negative.

1. Let's start from the phylum directory:

$ cd phylum

2. Run the account-functions test that demonstrates a negative account balance of -25.

$ make test

# Output:
...
    --- PASS: TestFile_utils/account-functions (0.04s)

3. Update the test to ensure that the account balance is never negative:

diff --git a/phylum/utils_test.lisp b/phylum/utils_test.lisp
index f859e17..f789dbe 100644
--- a/phylum/utils_test.lisp
+++ b/phylum/utils_test.lisp
@@ -13,7 +13,6 @@
     (assert-equal 50 (get (create-account! person1 50) "current_balance"))
     (assert-equal 100 (get (get-account person0) "current_balance"))
     (assert (account-transfer! person0 person1 25))
-    (assert (account-transfer! person1 person0 100))
-    (assert (account-do person0 account-found?))
-    (assert-equal -25 (account-do person1 get-balance))
-    (assert-equal 175 (account-do person0 get-balance))))
+    (assert-not (account-transfer! person1 person0 100))
+    (assert-equal 75 (get (get-account person0) "current_balance"))
+    (assert-equal 75 (get (get-account person1) "current_balance"))))

4. The test now fails because the transfer still succeeds:

$ make test

# Output:
...
 lisptest.go:252: utils_test.lisp:7: lisp:assert: the expressions is not falsey

5. Let's add a rule to prohibit negative account balances:

diff --git a/phylum/utils.lisp b/phylum/utils.lisp
index 531ce1a..9f92528 100644
--- a/phylum/utils.lisp
+++ b/phylum/utils.lisp
@@ -47,7 +47,6 @@
                              (put-account! acct-id balance)))))
 
         ;; account-transfer! moves amount units between two accounts.
-        ;; account-transfer! will allow account balances to go negative.
         (defun account-transfer! (from-id to-id amount)
           (account-do from-id
                       (lambda (from-found? from-bal)
@@ -55,6 +54,7 @@
                              (account-do to-id
                                          (lambda (to-found? to-bal)
                                            (and to-found?
+                                                (>= (- from-bal amount) 0)
                                                 (put-account! from-id (- from-bal amount))
                                                 (put-account! to-id (+ to-bal amount)))))))))

6. And re-run the test to confirm the test now passes:

$ make test

# Output:
...
--- PASS: TestFile_utils (0.06s)
    --- PASS: TestFile_utils/$load (0.03s)
    --- PASS: TestFile_utils/account-functions (0.03s)
PASS

7. 🎉 Congratulations! You fixed your first bug.

Add a new unit test

1. Let's add a new elps unit test to avoid creation of accounts with negative balances:

diff --git a/phylum/utils_test.lisp b/phylum/utils_test.lisp
index f789dbe..0db4c79 100644
--- a/phylum/utils_test.lisp
+++ b/phylum/utils_test.lisp
@@ -16,3 +16,7 @@
     (assert-not (account-transfer! person1 person0 100))
     (assert-equal 75 (get (get-account person0) "current_balance"))
     (assert-equal 75 (get (get-account person1) "current_balance"))))
+
+(test "no-negative-accounts"
+  (let ([person0 "person0"])
+    (assert-not (create-account! person0 -100))))

2. Run the new unit test to confirm that it fails:

$ make test

# Output:
...
    lisptest.go:252: utils_test.lisp:21: lisp:assert: the expressions is not falsey
                expression: (create-account! "person0" -1)
                    result: (sorted-map "account_id" "person0" "current_balance" -1)

3. Let's now add a check to prevent creating negative accounts:

diff --git a/phylum/utils.lisp b/phylum/utils.lisp
index 531ce1a..610dede 100644
--- a/phylum/utils.lisp
+++ b/phylum/utils.lisp
@@ -27,7 +27,8 @@
         (defun create-account! (acct-id balance)
           (account-do acct-id
                       (lambda (found? _)
                         (and (not found?)
+                             (>= balance 0)
                              (put-account! acct-id balance)
                              (make-account acct-id balance)))))

4. Run the tests now to see if they pass:

$ make test

# Output
...
--- PASS: TestFile_utils (0.09s)
    --- PASS: TestFile_utils/$load (0.03s)
    --- PASS: TestFile_utils/account-functions (0.03s)
    --- PASS: TestFile_utils/no-negative-accounts (0.02s)
PASS

5. You can run an individual test as follows, where --run takes Go-style pattern matches to run tests (see go test).

$ $(make echo:SHIRO_TEST) --run "/no-negative-accounts"
# Output:
...
--- PASS: TestFile_utils (0.06s)
    --- PASS: TestFile_utils/$load (0.03s)
    --- PASS: TestFile_utils/no-negative-accounts (0.02s)

NOTE: the account-functions test are skipped.
NOTE: The $load test automatically runs on every run.