formatting and more explanation for error

eeps/eep-0073.md

@@ -12,7 +12,7 @@ EEP 73: Zip generator
 Abstract
 ========
 
-This EEP proposes the addition of zip generators with a syntax of `&&` for 
+This EEP proposes the addition of zip generators with a syntax of `&&` for
 comprehensions in Erlang. The idea and syntax of zip generators (comprehension
 multigenerators) was first brought up by EEP-19. Even if the syntax and
 usages of zip generators proposed by this EEP is mostly the same with EEP-19,
@@ -72,11 +72,10 @@ produces a binary, etc.
     <<Expression || Qualifier(s)>>                   %% Binary Comprehension
     #{Expression || Qualifier(s)}                    %% Map Comprehension
 
-
 Qualifiers can have the following kind: filter, list generator, bitstring
 generator, and map generator. Except for filters, the other three kinds of
 qualifiers are generators. Their names refer to the type on the right hand
-side of `<-` or `<=`. Generators have the following form: 
+side of `<-` or `<=`. Generators have the following form:
 
     Pattern <- List                                  %% List Generator
     Pattern <= Bitstring                             %% Bitstring Generator
@@ -138,12 +137,14 @@ Generators of Different Lengths
 `lists:zip/2` and `lists:zip3/3` will fail if the given lists are not of the
 same length, where `zip/n` will also crash. Therefore, a zip generator raises a
 `bad generators` error when it discovers that the given generators are of
-different lengths. 
+different lengths.
 
 When a zip generator crashes because the containing generators are of
-different lengths, the error message is a tuple, where the first element
-is the atom `bad_generators`, and the second element is a tuple that contains
-the remaining data from all generators. 
+different lengths, the internal error message is a tuple, where the first
+element is the atom `bad_generators`, and the second element is a tuple that
+contains the remaining data from all generators. The user-facing error message
+is `bad generators: `, followed by the tuple containing remaining data from
+all generators.
 
 For example, this comprehension will crash at runtime. 
 
@@ -158,7 +159,7 @@ On the compiler's side, it is rather difficult to return the original zip
 generator in the error message, or to point out which generator is of
 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. 
+compiler or runtime.
 
 Non-generator in a Zip Generator
 -----------------
@@ -169,7 +170,7 @@ possible to catch such an error at compile-time, this error is caught by
 the Erlang linter.
 
 For example, the zip generator in the following comprehension contains a
-non-generator.
+filter.
 
     zip() -> [{X,Y} || X <- [1,2,3] && Y <- [1,2,3] && X > 0].
 
@@ -180,7 +181,6 @@ allowed in a zip generator, together with the position of the non-generator.
     %    6|  zip() -> [{X,Y} || X <- [1,2,3] && Y <- [1,2,3] && X > 0].
     %     |                                                       ^
 
-
 Backwards Compatibility
 ========================
 
@@ -190,7 +190,7 @@ this addition.
 Reference Implementation
 ========================
 
-[Git Branch][1] contains the implementation for zip generators. 
+[Git Branch][1] contains the implementation for zip generators.
 
 [1]: https://github.com/lucioleKi/otp/tree/isabell/compiler/zip-comprehension/OTP-19184
 [2]: https://downloads.haskell.org/~ghc/5.00/docs/set/parallel-list-comprehensions.html