[expect] adding stringStarts and docs

.semver

@@ -1,6 +1,6 @@
 ---
 :major: 1
 :minor: 0
-:patch: 11
+:patch: 12
 :special: ''
 :metadata: ''

Expecto/Expect.fs

@@ -144,9 +144,18 @@ let sequenceEqual (actual : _ seq) (expected : _ seq) format =
                       format i (ei.Current)
     i <- i + 1
 
-/// Ensures that the subject string contains the given substring. Otherwise
-/// fails with the passed message.
+/// Expect the string `subject` to contain `substring` as part of itself.
+/// If it does not, then fail with `format` and `subject` and `substring`
+/// as part of the error message.
 let stringContains (subject : string) (substring : string) format =
   if not (subject.Contains(substring)) then
     Tests.failtestf "%s. Expected subject string '%s' to contain substring '%s'."
                     format subject substring
+
+/// Expect the string `subject` to start with `prefix`. If it does not
+/// then fail with `format` as an error message together with a description
+/// of `subject` and `prefix`.
+let stringStarts (subject : string) (prefix : string) format =
+  if not (subject.StartsWith prefix) then
+    Tests.failtestf "%s. Expected subject string '%s' to start with '%s'."
+                    format subject prefix

README.md

@@ -141,7 +141,12 @@ This module is your main entry-point when asserting.
  - `isFalse`
  - `isTrue`
  - `sequenceEqual`
- - `stringContains`
+ - `stringContains` – Expect the string `subject` to contain `substring` as part
+   of itself.  If it does not, then fail with `format` and `subject` and
+   `substring` as part of the error message.
+ - `stringStarts` – Expect the string `subject` to start with `prefix` and if it
+   does not then fail with `format` as an error message together with a
+   description of `subject` and `prefix`.
  - `contains : 'a seq -> 'a -> string -> unit`
 
 ## `main argv` – how to run console apps