Make assignment (match) in comprehension work as strict generator

[richcarl]

It would be useful to be able to easily bind variables in the qualifiers of a comprehension, for example:

[Z || X <- Ls,
      {foo, Y} = g(X),
      Z <- f(Y, h(Y))].

You can do this today by writing a singleton generator:

[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; and someone might edit the code later and accidentally add extra elements to the right hand side, causing unintended Cartesian combinations.

It is in fact already allowed syntactically to have a = match expression in the qualifiers, but this is just interpreted as any other expression - thus expected to produce a boolean value, and if false, the current element will be skipped. Hence, any qualifier to the right of a V = Expr match will only execute if V is true. We can therefore expect that no such uses exist in practice. (The OTP code base has been checked and does not contain any.) To be completely safe, matches in a comprehension qualifier position can first be made an error, and then allowed with these binding semantics in the following major release.

This is related but orthogonal to #9134. It would make those few cases where variables have been exported from subexpressions of qualifiers much nicer to rewrite than requiring them to be singleton generators.

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CT Test Results

    2 files    332 suites   10m 23s ⏱️
  834 tests   831 ✅ 2 💤 1 ❌
5 581 runs  5 578 ✅ 2 💤 1 ❌

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[richcarl]

Correction: 1 occurence in the OTP test suites, in singleton_inference.erl, which is weird enough that I won't even try to rewrite it myself. Maybe @jhogberg who wrote it (commit ebc461e) can help?