Changes between Version 1 and Version 2 of Commentary/Compiler/UnusedImports

Timestamp:

06/17/09 01:56:13 (4 years ago)

Author:

simonpj

Comment:

--

Commentary/Compiler/UnusedImports

@@ -4 +4 @@
 flags, including #1148, #2267, #1074, #2436. 
 This page describes a new design.
-
-The idea is that for each use of an imported name, we will attribute
-that use to one or more import decls. Then, any import decls with no
-uses attributed to them are unused, and are warned about.
 
 == The current story == 
@@ -71 +67 @@
  * It's not worth trying to be too subtle.  The 90% case is very simple.
 
-Here is an attempt at a specification:
- 1.  Say that an ''import-item'' is either an entire import-all decl (eg `import Foo`),
- or a particular item in an import list (eg `import Foo( ..., x, ...)`).  
- 2.  For every `RdrName` in the program text, choose one of the import-items
- that brought it into scope, the "chosen import-item",
- and mark it "used".  
+Say that an ''import-item'' is either an entire import-all decl (eg `import Foo`),
+or a particular item in an import list (eg `import Foo( ..., x, ...)`).  
+The general idea is that for each use of an imported name, we will attribute
+that use to one (or possibly more) import-items. Then, any import items with no
+uses attributed to them are unused, and are warned about.
+More precisely:
+
+ 1.  For every `RdrName` in the program text, find all the import-items that brought it     into scope.  The lookup mechanism on `RdrNames` already takes account of whether the `RdrName` was qualified, and which imports have the right qualification etc, so this step is very easy.
+
+ 2. Choose one of these, the "chosen import-item", and mark it "used".  
+
  3.  Now bleat about any import-items that are unused.  For a decl
  `import Foo(x,y)`, if both the `x` and `y` items are unused, it'd be better
  to bleant about the entire decl rather than the individual items.
-
-In step 2, the lookup mechanism on `RdrNames` already takes account of whether
-the `RdrName` was qualified, and which imports have the right qualification etc.
 
 The import-item choosing step 2 implies that there is a total order on 
@@ -91 +89 @@
  * Otherwise choose the textually first one.
 
-Note that this algorithm chooses exactly one import-item in step 2.  It would
-also be sound to choose more than one if there was a tie, but then completely-duplicate
-imports might not be reported.
+Other notes:
+ * The algorithm chooses exactly one import-item in step 2.  It would
+ also be sound to choose more than one if there was a tie, but then completely-duplicate
+ imports might not be reported.
 
-Note that if we have an import item `import Foo (Bar(bar))`, then
-it's marked as used if either `Bar` or `bar` are used.  We could have yet finer
-resolution and report even unused sub-items.
+ * Note that if we have an import item `import Foo (Bar(bar))`, then
+ it's marked as used if either `Bar` or `bar` are used.  We could have yet finer
+ resolution and report even unused sub-items.
+
+ * We should retain the special case of not warning about `import Foo ()`, which implies "instance declarations only".
 
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