Well, there need not be any ambiguity if you think of "B given A" as "execute A before B", and remember that "given" has a lower precedence than "for" (So [B given A for x in seq] is parsed as [(B given A) for x in seq]

Then

retval = [expr(name) given name=something(x) for x in seq]

Is:

retval = []

for x in seq:

name = something(x)

retval.append(expr(name))

And

retval = [expr(name, x) for x in seq given name=something]

Is:

retval = []

name = something

for x in seq:

retval.append(expr(name, x))

But this is probably not a great solution, as it forces you to mentally unwrap comprehensions in a strange order and remember a non-obvious precedence rule.

On the plus-side, it lets you initialize generators with in-loop updates (which cannot as far as I see be done nicely with ":="):

retval = [expr(name, x) given name=update(name, x) for x in seq given name=something]

Is:

retval = []

name = something

for x in seq:

name = update(name, x)

retval.append(expr(name, x))

On Thu, May 31, 2018 at 10:44 AM, Neil Girdhar <mistersheik@gmail.com> wrote:

Yes, you're right. That's the ambiguity I mentioned in my last message. It's too bad because I want given for expressions and given for comprehensions. But if you have both, there's ambiguity and you would at least need parentheses:

[(y given y=2*x) for x in range(3)]

That might be fine.

On Thu, May 31, 2018 at 4:34 AM Peter O'Connor <peter.ed.oconnor@gmail.com> wrote:
* Sorry, message sent too early:

On Thu, May 31, 2018 at 4:50 AM, Neil Girdhar <mistersheik@gmail.com> wrote:

[expression given name=something for x in seq]

retval = []
name = something
for x in seq:
retval.append(expression)
return retval

That's a little confusing then, because, given the way given is used outside of comprehensions, you would expect

[y given y=2*x for x in range(3)]

to return [0, 2, 4], but it would actually raise an error.

On Thu, May 31, 2018 at 10:32 AM, Peter O'Connor <peter.ed.oconnor@gmail.com> wrote:

On Thu, May 31, 2018 at 4:50 AM, Neil Girdhar <mistersheik@gmail.com> wrote:

[expression given name=something for x in seq]

retval = []
name = something
for x in seq:
retval.append(expression)
return retval

That's a little strange confusing then, because, given the way given is used outside of comprehensions, you would expect

for x in range(3):
y given y=2*x

[y given y=2*x for x in range(3)]

to return [0, 2, 4], but it would actually raise an error.