F# beginner using let on multiple dependent names - f#

Question

I hate to ask such a noob question, but I have hunted high and low online--and this is literally my first day.
The following VBA excerpt assigns two variables, where the first variable feeds into the second variable (this code is from www.spreadsheetadvice.com):
If zabs <= 12.7 Then
...
If zabs <= 1.28 Then
Temp = y + a3 - a4 / (y + a5 + a6 / (y + a7))
Q = a0 - zabs * (a1 - a2 * y / Temp)
Else
Temp = (zabs - b5 + b6 / (zabs + b7 - b8 / (zabs + b9 + b10 / (zabs + b11))))
Q = pdF / (zabs - b1 + (b2 / (zabs + b3 + b4 / Temp)))
End If
Else ...
I'm trying to replicate it in F#--without using mutables. This works:
let Temp =
if zabs <= 1.28 then
y + a3 - a4 / (y + a5 + a6 / (y + a7))
elif zabs <= 12.7 then
(zabs - b5 + b6 / (zabs + b7 - b8 / (zabs + b9 + b10 / (zabs + b11))))
else
0.0
let Q =
if zabs <= 1.28 then
a0 - zabs * (a1 - a2 * y / Temp)
elif zabs <= 12.7 then
pdF / (zabs - b1 + (b2 / (zabs + b3 + b4 / Temp)))
else
0.0
But I don't like separating the two related variables--or repeating the If statements. This doesn't work but is as close as I've come:
let Temp, Q =
if zabs <= 1.28 then
y + a3 - a4 / (y + a5 + a6 / (y + a7)),
a0 - zabs * (a1 - a2 * y / Temp)
elif zabs <= 12.7 then
(zabs - b5 + b6 / (zabs + b7 - b8 / (zabs + b9 + b10 / (zabs + b11)))),
pdF / (zabs - b1 + (b2 / (zabs + b3 + b4 / Temp)))
else
0.0,
0.0
It took me a while to figure out the commas on the 3rd, 6th, and 9th lines (if they are in fact right). But the compiler thinks I am trying to use Temp before it is assigned. I assume, or hope, I'm just missing a simple idiom.

Answer 1

This works:
let Temp, Q =
if zabs <= 1.28 then
let temp = y + a3 - a4 / (y + a5 + a6 / (y + a7))
let q = a0 - zabs * (a1 - a2 * y / temp)
temp, q
elif zabs <= 12.7 then
let temp = (zabs - b5 + b6 / (zabs + b7 - b8 / (zabs + b9 + b10 / (zabs + b11))))
let q = pdF / (zabs - b1 + (b2 / (zabs + b3 + b4 / temp)))
temp, q
else
0.0, 0.0

Answer 2

It took me a while to figure out the commas on the 3rd, 6th, and 9th lines (if they are in fact right). But the compiler thinks I am trying to use Temp before it is assigned. I assume, or hope, I'm just missing a simple idiom.
Yes, you are missing the basics of the language, you must put the commas because you declare a tuple so you have to bind a value of type 'a * 'b
I suggest to use pattern matching, one of the more powerful thing of F# (and other functional languages) - https://msdn.microsoft.com/en-us/library/dd547125.aspx
let yourfunc zabs =
match zabs with
| x when x <= 1.28 -> let temp = y + a3 - a4 / (y + a5 + a6 / (y + a7))
let q = a0 - zabs * (a1 - a2 * y / temp)
temp, q
| x when x <= 12.7 -> let temp = (zabs - b5 + b6 / (zabs + b7 - b8 / (zabs + b9 + b10 / (zabs + b11))))
let q = pdF / (zabs - b1 + (b2 / (zabs + b3 + b4 / temp)))
temp, q
| _ -> 0.0,0.0

Answer 3

I think I was making a simple mistake: because Temp is temporary and subordinate to Q, then Temp should be declared with a scope local to the declaration of Q, not with equal scope as Q. With that figured out, I think using If and using Match work equally well. But Match is probably more idiomatic (thanks Alessandro). Thusly:
let Q =
match zabs with
| i when i <= 1.28 ->
let Temp = y + a3 - a4 / (y + a5 + a6 / (y + a7))
a0 - zabs * (a1 - a2 * y / Temp)
| i when i <= 12.7 ->
let Temp = (zabs - b5 + b6 / (zabs + b7 - b8 / (zabs + b9 + b10 / (zabs + b11))))
pdF / (zabs - b1 + (b2 / (zabs + b3 + b4 / Temp)))
| _ ->
0.0
So now I'm bound to want to make two declarations that do need to be of the same scope some time soon and come back to a similar situation again...