How to make a value_of_things function f# - f#

Question

Hey guys let's say I have a function that gets that day's rate of how much something costs, by the pound, and multiplies it by how many pounds a customer wants. i.e
let convert_func (crab_rate, lobster_rate);
//and then on a certain day it is
let (crab_rate, lobster_rate) = convert_fun(3.4, 6.8); // $3.8 a pound for crab, $6.8 a pound for lobster.
// 10 being how many pounds i want.
crab_rate10 ;
Then my out put would be whatever 38 since ($3.8 * 10lbs) = $38
I tried doing if statements so that when the user just wants to find out the total cost of one thing and not both. But I keep getting errors. I can't figure out how to store the rate values in the parameters and then calling the function.
This is what i tried
let crab_rate (pound, rate) = (float pound) * rate;
let lobster_rate (pound, rate) = (float pound) * rate;
let convert_func (crab_rate, lobster_rate)= function (first,second ) ->
if crab_rate then (float pound) * rate;
elif lobster_rate (float pound) * rate;
let (crab_rate, lobster_rate) = convert_fun(3.4, 6.8); // $3.8 a pound for crab, $6.8 a pound for lobster.
// 10 being how many pounds i want.
crab_rate10 ;

Answer 1

I think you should start by making a general function for converting a cost/weight and a weight into a cost. In F#, you can even use units of measure to help you:
[<Measure>] type USD // Unit of measure: US Dollars
[<Measure>] type lb // Unit of measure: lbs
let priceForWeight rate (weight : float<lb>) : float<USD> =
rate * weight
The nice thing about functional languages with curried arguments is that we can easily use partial function application. That means when we have a function that has two arguments, we can choose to supply just one argument and get back a new function from that one remaining argument to the result.
We can therefore define a further pair of functions that use this priceForWeight function.
let crabPriceForWeight weight = priceForWeight 3.8<USD/lb> weight
let lobsterPriceForWeight weight = priceForWeight 6.8<USD/lb> weight
Notice that we've used our original function to define two new functions with fixed rates.
We can then evaluate it like this:
let crabPrice10 = crabPriceForWeight 10.0<lb> // result 38.0<USD>
Of course, you can also define a function that returns both prices together as a tuple for a supplied weight:
let crabAndLobsterPriceForWeight weight =
crabPriceForWeight weight, lobsterPriceForWeight weight