Merge pull request #74 from richcarl/comprehension-assign

EEP-77: comprehension assignment

Author: kikofernandez

eeps/eep-0077.md

@@ -0,0 +1,121 @@
+    Author: Richard Carlsson <carlsson.richard(at)gmail(dot)com>
+    Status: Draft
+    Type: Standards Track
+    Created: 10-Jan-2025
+    Erlang-Version: OTP-28.0
+    Post-History:
+****
+EEP 77: Assignment in Comprehensions
+----
+
+Abstract
+========
+
+This EEP adds assignments to comprehension qualifier lists, providing
+a convenient and readable alternative to other syntactical tricks.
+
+Rationale
+=========
+
+It would often be useful to be able to easily bind variables in the
+qualifier sequence of a comprehension, for example:
+
+```erlang
+[Z || X <- Ls,
+      {foo, Y} = g(X),
+      Z <- f(Y, h(Y))].
+```
+
+using plain `Pattern = ...,` entries between qualifiers.
+
+You can do this today by writing a singleton generator:
+
+```erlang
+[Z || X <- Ls,
+      {foo, Y} <- [g(X)],  % produce single element
+      Z <- f(Y, h(Y))].
+```
+
+but this has some drawbacks:
+
+- The intent is not clear to the reader
+- You have to remember to write a single element list around the
+  right hand side argument (which itself could be a list)
+- You probably want it to be a strict generator so typos don't just
+  silently yield no elements
+- Someone editing the code later may accidentally add extra
+  elements to the right hand side list, causing unintended Cartesian
+  combinations
+
+Another trick is to piggy-back on a boolean test, using export of
+bindings from within a subexpression to propagate to the subsequent
+qualifiers:
+
+```erlang
+[Z || X <- Ls,
+      is_tuple(Y = g(X)),
+      Z <- f(Y, h(element(2,Y)))].
+```
+
+which is very much not a recommended style in Erlang, making it hard
+to see where the bindings come from. It also relies on having a
+suitable test as part of the qualifiers for the value you want to
+bind. If you don't, it is possible to invent a dummy always-true test:
+
+```erlang
+[Z || X <- Ls,
+      foobar =/= (Y = g(X)),
+      Z <- f(Y, h(Y))].
+```
+
+Such tricks are very bad for readability and maintenance of the code,
+making the logic hard to follow.
+
+Specification
+========================
+
+It is in fact already allowed syntactically to have a `Pattern = ...`
+match expression in the qualifiers. This however gets interpreted as
+any other expression - thus expected to produce a boolean value - and
+if `false`, the current element will be skipped.
+
+Hence, any qualifier following after a match `Var = Expr` will only be
+executed if `Var` has the value `true`. We can therefore expect that
+no such uses exist in practice, because `Var` would be fully
+redundant. (The OTP code base has been checked and does not contain
+any.) To avoid incompatibilities with any possible existing code that
+relies on the current behaviour, the new semantics can be implemented
+as an optional feature at first, and an error be reported for such
+assignments when the feature flag is not enabled.
+
+The main change needed is then for the compiler to detect a match
+expression `Pattern = Expr` in the qualifier list, and treat it as a
+strict singleton generator `Pattern <-:- [Expr]`.
+
+Reference Implementation
+------------------------
+
+A [reference implementation][GitHub branch] exists in the
+`lc-match-operator` branch of the author's GitHub account, together with a
+[GitHub pull request][GitHub PR] to the Erlang/OTP repository.
+
+[GitHub branch]: https://github.com/richcarl/otp/tree/lc-match-operator
+    "Reference implementation branch on GitHub"
+
+[GitHub PR]:  https://github.com/erlang/otp/pull/9153
+    "GitHub Pull Request"
+
+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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