F# do let execution order

Question

I'm new to F# so this block of code seems strange to me

let randomTest avgWait avgBusyTime numExp numClients labsRules =
    let clients, _ = mkClientsAndLabs numClients labsRules 
    doTest [for i in 0..numClients-1 -> randomTestClient clients i avgWait avgBusyTime numExp  ]

do let clients, _ = mkClientsAndLabs 5 [rulesA; rulesB] 
   doTest [scheduledClient clients 0 [(0, 500, A)];     // Request a lab at the very start, use for "A" for 0.5 seconds
           scheduledClient clients 1 [(200, 300, Mix (Mix (A,Mix (A,A)),B))] ;   // Request after 0.2s, release 0.3s later.

           scheduledClient clients 2 [(300, 200, Mix (A,Mix (A,A)))];   // These three will all be waiting for a lab.
           scheduledClient clients 3 [(400, 200, Mix (A,A))];           // Client 2 should include the others as guests.
           scheduledClient clients 4 [(400, 200, A)]
          ]

What I am unsure is the do let declaration - it obviously is declared after randomTest yet randomTest can still call that function. What is the order of this code execution?

Answer 1

The way it's written may be confusing. There is no such thing as a do let declaration. In fact it's a whole do {code} block with a let binding inside the {code} . That means it's not a function declaration, a do block is just a code to be executed, it does not declare functions or values.

Should be easier to read like this:

do 
   let clients, _ = mkClientsAndLabs 5 [rulesA; rulesB] 
   doTest [scheduledClient clients 0 [(0, 500, A)];     // Request a lab at the very start, use for "A" for 0.5 seconds
           scheduledClient clients 1 [(200, 300, Mix (Mix (A,Mix (A,A)),B))] ;   // Request after 0.2s, release 0.3s later.

           scheduledClient clients 2 [(300, 200, Mix (A,Mix (A,A)))];   // These three will all be waiting for a lab.
           scheduledClient clients 3 [(400, 200, Mix (A,A))];           // Client 2 should include the others as guests.
           scheduledClient clients 4 [(400, 200, A)]
          ]

So the order of execution is first let randomTest ... , then the do block.