Updated EEP for behaviors with strict generators

Author: lucioleKi

eeps/eep-0073.md

@@ -1,5 +1,5 @@
     Author: Isabell Huang <isabell(at)erlang(dot)org>
-    Status: Draft
+    Status: Final/28.0 Implemented in OTP release 28
     Type: Standards Track
     Created: 21-Sep-2024
     Erlang-Version: OTP-28.0
@@ -73,7 +73,7 @@ list of tuples. Variable bindings and pattern matching within a zip generator
 works as expected, as `Nm` is supposed to bind to the same value in `{Nm,P}`
 and `{Nm,S}`. If the binding fails, then one element from each of the 3
 generators is skipped. (If a strict generator is used, then the comprehension
-fails with exception `badmatch`, as specified in EEP-70.)
+fails with an exception, as specified in [Error Behaviors](#error-behaviors).)
 
 In summary, zip generators remove the user's need to call the zip function
 within comprehensions and allows for any number of lists to be zipped at once.
@@ -138,6 +138,23 @@ filtered out from the resulting list.
 
     [{X, Y} || X <- [a, b, c] && <<Y>> <= <<2, 4, 6>>, Y =/= 2].
 
+The skipping behavior of individual generators within a zip generator is always
+respected. When strict generators are present in a zip generator, during every
+round of evaluation, all strict generators will be evaluated even if another
+relaxed generator already causes the result to be skipped.
+
+For example, the following comprehension has a zip generator containing a
+strict generator and a relaxed generator.
+
+    [{X, Y} || {a, X} <:- L1 && {b, Y} <- L2]
+
+It will `badarg` if pattern matching for `{a, X}` fails, for example, when
+`L1 = [{a, 1}, {bad, 2}, {a, 3}]`. It will simply skip 1 item from `L1` and
+1 item from `L2` if pattern matching for `{b, Y}` fails, for example, when
+`L2 = [{b, 1}, {bad, 2}, {b, 3}]`. The attempt of matching `{a, X}` will
+be made every round, regardless of generators ordering and whether other
+pattern matchings succeed.
+
 Comparing to using helper functions, there is one advantage of using a zip
 generator: The Erlang compiler does not generate any tuple when a zip
 generator is translated into core Erlang. The generated code reflects the
@@ -170,7 +187,7 @@ all generators.
 
 For example, this comprehension will crash at runtime.
 
-     [{X,Y} || X <- [1,2,3] && Y <- [1,2,3,4]].
+    [{X,Y} || X <- [1,2,3] && Y <- [1,2,3,4]].
 
 The resulting error tuple is `{bad_generators,{[],[4]}}`. This is because
 when the comprehension crashes, the first list in the zip generator has
@@ -183,6 +200,35 @@ different length comparing to others. The proposed error message aims to
 gives the most helpful information without imposing extra burden on the
 compiler or runtime.
 
+Failed Strict Generator in a Zip Generator
+-----------------
+
+When a zip generator crashes because at least one strict generators contained
+in it fails, the resulting error tuple is of the same format as when generators
+are of different lengths. Its first element is the atom `bad_generators`, and
+the second element is a tuple containing remaining data from all generators.
+
+For example, this comprehension will crash at runtime, because `bad` cannot
+match the pattern `{ok,A}`.
+
+    [A + B || {ok,A} <:- [bad, {ok,1}] && B <- [2,3]].
+
+The resulting error tuple is `{bad_generators,{[bad, {ok,1}],[2,3]}}`. Although
+strict generators alone fail with exception `badmatch`, as specified in
+[EEP-70][4], it is not plausible to use the same exception in zip generators,
+due to difficulty in distinguishing between `badmatch` and `bad_generators`
+errors.
+
+In the following example, the comprehension will crash at runtime, either for
+the failed strict generator, or for two generators of different lengths.
+
+    [A + B || {ok,A} <:- [bad] && B <- []].
+
+The emitted error message is `{bad_generators,{[bad],[]}}`. We do not
+distinguish between the two errors, and instead always output all remaining
+data in the generators. The user can examine the remaining data and see that
+the first generator fails matching, and the second generator is empty.
+
 Non-generator in a Zip Generator
 -----------------
 
@@ -212,8 +258,7 @@ this addition.
 Reference Implementation
 ========================
 
-[compiler: Add zip generators for comprehensions][1] contains the implementation
-for zip generators.
+[PR-8926][1] contains the implementation for zip generators.
 
 [1]: https://github.com/erlang/otp/pull/8926
 [2]: https://downloads.haskell.org/~ghc/5.00/docs/set/parallel-list-comprehensions.html