[erlang-questions] Compiler list comprehension optimizations

Jonathan Leivent jleivent@REDACTED
Fri Jun 14 03:38:15 CEST 2013

I have some questions about compiler optimizations on list comprehensions.

The efficiency guide says that list comprehensions that are "neither 
assigned to a variable, nor passed to another function, nor returned" 
will be optimized by the compiler to avoid the list construction.

However, Dialyzer sometimes complains about such list comprehensions 
with "Expression produces a value of type ['ok'], but this value is 
unmatched".  So I have developed the habit of assigning to "_", as with:

foo(...) ->
   _ = [bar(X) || ...],

After re-reading that line in the efficiency guide, it seems like this 
assignment might actually defeat the above compiler optimization.  Does 
it?  Or does the compiler still manage to do the optimization?

If this does prevent the optimization, is there a way to get the 
optimization and prevent the Dialyzer warning while still using a list 

Another question about list comprehension optimizations:  If a list 
being iterated over is generated from a function that just builds the 
list element-wise, can the compiler inline that function so that the 
intermediate list isn't built?  For instance, if the function is 
something like the very useful list of all non-empty tails of a list:

   alltails(L=[_|T]) -> [L|alltails(T)];
   alltails([]) -> [].

   [... || T <- alltails(L), ...]

can the compiler generate code that avoids actually building the list of 
all tails of L?

Obviously, one could write alltails in (reversed) tail-recursive form, 
but I would think that if the compiler has any chance of inlining 
alltails into that list comprehension to remove the intermediate list, 
the best chance would be with the simpler non-tail-recursive form.

-- Jonathan

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