Feature Request: filtering in comprehensions

[kmsquire]

Comprehensions' offer an easy-to-understand paradigm for map-like operations:

bases = ['A', 'C', 'G', 'T']
basemap = map(i -> bases[i & 3 + 1], [0:255])
basemap2 = [ bases[i & 3 + 1] for i in 0:255 ]
assert(basemap == basemap2)

Some languages (Python, Scala, at least) add syntax to comprehensions to allow filtering of output values:

# Python
xs = range(1,101)
prime_100 = filter(isprime, xs)  # assumes isprime is defined
prime_100_2 = [x for x in xs if isprime(x)]
assert(prime_100 == prime_100_2)

It would be nice if julia supported a similar syntax.

[JeffBezanson]

Dup of #550.

[kmsquire]

Sorry, need to search the issues more carefully...