Increase test coverage (#3)

* Checkpoint.

* Checkpoint.

* Remove Checker.Interfaces. More tests.

* A few more tests and a fix for a bug that they turned up.

* Expand Readme.

* More test coverage, and fix a test bug.

Readme.md

@@ -16,6 +16,96 @@ if it has a more-specific type than it actually needs.
 For example,
 if your function takes a `*os.File` parameter,
 but it’s only ever used for its `Read` method,
-it could be specified as an abstract `io.Reader` instead,
-making it possible to use the function
-on a much wider variety of concrete types.
+it could be specified as an abstract `io.Reader` instead
+
+## Why decouple?
+
+When you decouple a function parameter from its too-specific type,
+you broaden the set of values on which it can operate.
+
+You also make it easier to test.
+For a simple example,
+suppose you’re testing this function:
+
+```go
+func CountLines(f *os.File) (int, error) {
+  var result int
+  sc := bufio.NewScanner(f)
+  for sc.Scan() {
+    result++
+  }
+  return result, sc.Err()
+}
+```
+
+Your unit test will need to open a testdata file and pass it to this function to get a result.
+But as `decouple` can tell you,
+`f` is only ever used as an `io.Reader`
+(the type of the argument to [bufio.NewScanner](https://pkg.go.dev/bufio#NewScanner)).
+
+If you were testing `func CountLines(r io.Reader) (int, error)` instead,
+the unit test can simply pass it something like `strings.NewReader("a\nb\nc")`.
+
+## Installation
+
+```sh
+go install github.com/bobg/decouple/cmd/decouple@latest
+```
+
+## Usage
+
+```sh
+decouple [-v] [DIR]
+```
+
+This produces a report about the Go packages rooted at DIR
+(the current directory by default).
+With -v,
+very verbose debugging output is printed along the way.
+
+The report will be empty if decouple has no findings.
+Otherwise, a report will look something like this:
+
+```
+$ decouple
+/home/bobg/kodigcs/handle.go:105:18: handleDir
+    req: [Context]
+    w: io.Writer
+/home/bobg/kodigcs/handle.go:167:18: handleNFO
+    req: [Context]
+    w: [Header Write]
+/home/bobg/kodigcs/handle.go:428:6: isStale
+    t: [Before]
+/home/bobg/kodigcs/imdb.go:59:6: parseIMDbPage
+    cl: [Do]
+```
+
+This is the output when running decouple on [the current commit](https://github.com/bobg/kodigcs/commit/f4e8cf0e44de0ea98fa7ad4f88705324ff446444)
+of [kodigcs](https://github.com/bobg/kodigcs).
+It’s saying that:
+
+- In the function [handleDir](https://github.com/bobg/kodigcs/blob/f4e8cf0e44de0ea98fa7ad4f88705324ff446444/handle.go#L105),
+  The `req` parameter is being used only for its `Context` method
+  and so could be declared as `interface{ Context() context.Context }`,
+  allowing objects other than `*http.Request` values to be passed in here;
+- Also in [handleDir](https://github.com/bobg/kodigcs/blob/f4e8cf0e44de0ea98fa7ad4f88705324ff446444/handle.go#L105),
+  `w` could be an `io.Writer`,
+  allowing more types to be used than just `http.ResponseWriter`;
+- Similarly in [handleNFO](https://github.com/bobg/kodigcs/blob/f4e8cf0e44de0ea98fa7ad4f88705324ff446444/handle.go#L167),
+  `req` is used only for its `Context` method,
+  and `w` for its `Write` and `Header` methods
+  (more than `io.Writer`, but less than `http.ResponseWriter`);
+- Anything with a `Before(time.Time) bool` method
+  could be used in [isStale](https://github.com/bobg/kodigcs/blob/f4e8cf0e44de0ea98fa7ad4f88705324ff446444/handle.go#L428),
+  it does not need to be limited to `time.Time`;
+- The `*http.Client` argument of [parseIMDbPage](https://github.com/bobg/kodigcs/blob/f4e8cf0e44de0ea98fa7ad4f88705324ff446444/imdb.go#L59)
+  is being used only for its `Do` method.
+
+Note that,
+in the report,
+the presence of square brackets means “this is a set of methods,”
+while the absence of them means “this is an existing type that already has the right method set”
+(as in the `io.Writer` line in the example above).
+Decouple can’t always find a suitable existing type even when one exists,
+and if two or more types match,
+it doesn’t always choose the best one.