Add proposal for non-skipping generators

eeps/eep-0070.md

@@ -0,0 +1,151 @@
+    Author: Dániel Szoboszlay <[email protected]>
+    Status: Draft
+    Type: Standards Track
+    Created: 01-Jul-2024
+    Erlang-Version: 28
+    Post-History:
+****
+EEP 70: Strict and relaxed generators
+----
+
+Abstract
+========
+
+This EEP proposes the addition of a new, *strict* variant of all
+existing generators (list, bit string and map).  Currently existing
+generators are *relaxed*: they ignore terms in the right-hand side
+expression that do not match the left-hand side pattern.  Strict
+generators on the other hand shall fail with exception `badmatch`.
+
+Rationale
+=========
+
+The motivation for strict generators is that relaxed generators
+can hide the presence of unexpected elements in the input data of a
+comprehension.  For example consider the below snippet:
+
+    [{User, Email} || #{user := User, email := Email} <- all_users()]
+
+This list comprehension would filter out users that don't have an email
+address.  This may be an issue if we suspect potentially incorrect input
+data, like in case `all_users/0` would read the users from a JSON file.
+Therefore cautious code that would prefer crashing instead of silently
+filtering out incorrect input would have to use a more verbose map
+function:
+
+    lists:map(fun(#{user := User, email := Email}) -> {User, Email} end,
+              all_users())
+
+Unlike the generator, the anonymous function would crash on a user
+without an email address.  Strict generators would allow similar
+semantics in comprehensions too:
+
+    [{User, Email} || #{user := User, email := Email} <:- all_users()]
+
+This generator would crash (with a `badmatch` error) if the pattern
+wouldn't match an element of the list.
+
+The proposed operators for strict generators are `<:-` (for lists
+and maps) and `<:=` (for bit strings) instead of `<-` and `<=`.  This
+syntax was chosen because `<:-` and `<:=` somewhat resemble the `=:=`
+operator that tests whether two terms match, and at the same time keep
+the operators short and easy to type. Having the two types of operators
+differ by a single character, `:`, also makes the operators easy to
+remember as "`:` means strict."
+
+Alternate Designs
+=================
+
+There are numerous other ways of achieving the goal of crashing upon
+encountering non-matching input:
+
+1. Converting the comprehension to a `map` call, just as in the example
+   of the previous section.
+
+2. Moving the pattern matching that may fail to the result expression of
+   the comprehension:
+
+    [begin
+         #{user:= User, email := Email} = Usr,
+         {User, Email}
+     end
+     || Usr <- all_users()]
+
+3. Moving the pattern matching that may fail to a filter in the
+   comprehension (which would still have to evaluate to a boolean,
+   making this solution quite awkward looking):
+
+    [{User, Email}
+     || Usr <- all_users(),
+        (#{user := User, email := Email} = Usr) > 0]
+
+4. The same as before, but using binders proposed in [EEP 12][]:
+
+    [{User, Email}
+     || Usr <- all_users(),
+        #{user := User, email := Email} = Usr]
+
+Most of these solutions are much more verbose than the strict
+generator syntax, undermining the compactness of comprehensions.  But
+there are more serious issues too:
+
+* 1 would not work for bit string comprehensions, because there isn't
+  a `map` function available for bit strings.
+
+* 2 would make it impossible to use sub terms of the pattern (`Usr` in
+  this example) in subsequent generators or filters.
+
+* The intent of the code isn't clear in most of these solutions
+  (definitely not in 3, and arguably in 4 and 2).
+
+* None of these techniques can solve the problem of final non-matching
+  bits of a bit string.
+
+The issue with bit string generators is that unlike lists and maps, bit
+strings don't have natural "elements" the generator could iterate over.
+Instead the pattern in the generator dictates where to split the bit
+string into parts. This could inevitably lead to situations where the
+bit string ends with some bits that don't match the pattern, like in
+this example:
+
+    [X || <<X:16>> <- <<1,2,3>>]
+
+The existing generator would skip these final non-matching bits, and
+this behaviour cannot be changed due to backward compatibility reasons.
+It is only possible to guarantee that a bit string generator would fully
+consume its input by introducing a new type of bit string generator or
+some other new syntax that would alter the behaviour of the existing
+generator.
+
+Reference Implementation
+========================
+
+Current implementation: [PR #8625][].
+
+Backward Compatibility
+======================
+
+There are no backward compatibility issues with the proposed new syntax,
+since the `<:-` and `<:=` operators used to be invalid syntax in
+previous versions of Erlang.
+
+[PR #8625]: https://github.com/erlang/otp/pull/8625
+    "Reference implementation PR"
+
+[EEP 12]: eep-0012.md
+    "Extensions to comprehensions, O'Keefe"
+
+Copyright
+=========
+
+This document is placed in the public domain or under the CC0-1.0-Universal
+license, whichever is more permissive.
+
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