Why does the FSI not always give me the expected ‘it’ value?

General
1

Note: I have seen this with some code but not others so I’m confused at to when to expect it happening.

I have some basic code in an FSX script that I am experimenting with:

let hasBreachedTarget target total = if total >= target then true else false 

let report hasBreached total = 
    if total |> hasBreached then Ok total else Error total 

let loadUp target collection = 

    let hasBreachedTarget = hasBreachedTarget target 

    let report = report hasBreachedTarget

    let rec load acc collection = 
        match collection with 
        | [] -> report acc
        | [single] -> report (acc + single)
        | head::tail -> 
            let newTotal = acc + head 
            if newTotal |> hasBreachedTarget then
                report newTotal 
            else 
                load newTotal tail

    load 0 collection 

let result = [ 1; 5; 4; 3; 4; 5 ] |> loadUp 15

…which, when executed in the FSI via Ctrl+A and Alt+Enter, reports what I would expect:

val hasBreachedTarget: target: 'a -> total: 'a -> bool when 'a: comparison
val report: hasBreached: ('a -> bool) -> total: 'a -> Result<'a,'a>
val loadUp: target: int -> collection: int list -> Result<int,int>
val result: Result<int,int> = Ok 17

However, if I change the last bit of code to simply (without the ‘let result =’):

[ 1; 5; 4; 3; 4; 5 ] |> loadUp 15

… I get this result:

val hasBreachedTarget: target: 'a -> total: 'a -> bool when 'a: comparison
val report: hasBreached: ('a -> bool) -> total: 'a -> Result<'a,'a>
val loadUp: target: int -> collection: int list -> Result<int,int>

…with no ‘it’ value.
I thought the ‘it’ value was supposed to be the result of the last thing executed.

If I try something even simpler, like:

[ 1; 5; 4; 3; 4; 5 ] |> List.sum

…I get an ‘it’ value, but I don’t get ‘it’ with my code above.

Am I doing something wrong, or misinterpreting something?
Is it something to do with the recursive nature of the loadUp function?

Note: Microsoft (R) F# Interactive version 12.5.0.0 for F# 7.0

2

Wow, I wonder if that’s a regression??
I have F# Interactive version 12.4.0.0 for F# 7.0, and I get the it value:

> let hasBreachedTarget target total = if total >= target then true else false
-
- let report hasBreached total =
-     if total |> hasBreached then Ok total else Error total
-
- let loadUp target collection =
-
-     let hasBreachedTarget = hasBreachedTarget target
-
-     let report = report hasBreachedTarget
-
-     let rec load acc collection =
-         match collection with
-         | [] -> report acc
-         | [single] -> report (acc + single)
-         | head::tail ->
-             let newTotal = acc + head
-             if newTotal |> hasBreachedTarget then
-                 report newTotal
-             else
-                 load newTotal tail
-
-     load 0 collection
- [ 1; 5; 4; 3; 4; 5 ] |> loadUp 15;;
val hasBreachedTarget: target: 'a -> total: 'a -> bool when 'a: comparison
val report: hasBreached: ('a -> bool) -> total: 'a -> Result<'a,'a>
val loadUp: target: int -> collection: int list -> Result<int,int>
val it: Result<int,int> = Ok 17
3

This sort of thing has come up often for me but I don’t know if it’s just me that’s doing something wrong.

For instance, if I reset the FSI session (not pressing Enter in the FSI window) and then Ctrl+A and Alt+Enter the following code (same code as earlier):

let hasBreachedTarget target total = 
    if total >= target then true else false 

let report hasBreached total = 
    if total |> hasBreached then Ok total else Error total 

let loadUp target collection = 

    let hasBreachedTarget = hasBreachedTarget target 

    let report = report hasBreachedTarget

    let rec load acc collection = 
        match collection with 
        | [] -> 
            report acc
        | [single] -> 
            report (acc + single)
        | head::tail -> 
            let newTotal = acc + head 
            if newTotal |> hasBreachedTarget then
                report newTotal 
            else 
                tail |> load newTotal 

    collection |> load 0  

[ 1; 5; 4; 3; 4; 5 ] |> loadUp 15

…I get this as the output:

Welcome to F# Interactive for .NET Core in Visual Studio. To execute code, either
  1. Use 'Send to Interactive' (Alt-Enter or right-click) from an F# script. The F# Interactive process will
     use any global.json settings associated with that script.
  2. Press 'Enter' to start. The F# Interactive process will use default settings.
> 

Microsoft (R) F# Interactive version 12.5.0.0 for F# 7.0
Copyright (c) Microsoft Corporation. All Rights Reserved.

For help type #help;;

> val it: unit = ()

> val hasBreachedTarget: target: 'a -> total: 'a -> bool when 'a: comparison
val report: hasBreached: ('a -> bool) -> total: 'a -> Result<'a,'a>
 list -> Result<int,int>

>

However, if I reset the session, press Enter in the FSI window (do I have to do that, and if so, why?), and then execute the code in the same way I get:

Welcome to F# Interactive for .NET Core in Visual Studio. To execute code, either
  1. Use 'Send to Interactive' (Alt-Enter or right-click) from an F# script. The F# Interactive process will
     use any global.json settings associated with that script.
  2. Press 'Enter' to start. The F# Interactive process will use default settings.
> 
Microsoft (R) F# Interactive version 12.5.0.0 for F# 7.0
Copyright (c) Microsoft Corporation. All Rights Reserved.

For help type #help;;

> val it: unit = ()

> 
val hasBreachedTarget: target: 'a -> total: 'a -> bool when 'a: comparison
val report: hasBreached: ('a -> bool) -> total: 'a -> Result<'a,'a>
-> collection: int list -> Result<int,int>

>

I don’t know where the val it: unit = () comes from (in either run), or why the type signature for the report function is different between runs – list versus collection.

I’ve also had times where putting a newline at the end of the source code gives a slightly different result than not putting a newline at the end.

I’ve also had times where the FSI seems to have ‘forgotten’ some values/functions that I haven’t changed in the source code and I’ve had to execute the whole thing again to ‘re-educate’ it.

I really like the FSI but it can be frustrating sometimes when I don’t know if I can trust the output and have to run the same thing more than once, but maybe slightly differently, just in case it didn’t give me ‘the truth’ the first time.

4

TBH, I don’t use the “integrated” FSI because I don’t really get it. I just use my terminal window and manually launch dotnet fsi, and then I’ve remapped <Alt>+<Enter> to send the highlighted code to my terminal. What I don’t like is when I do that I don’t get the nice window that shows you the values of all your current variables, but I really value just being able to understand everything. I guess what I’m saying is it sounds to me like your issues are with the integrated FSI, and I don’t think I’m the right person to help with that :frowning: .

5

Thanks for the extra information.

I’ve just run the code via VS Code and I got the result I was originally expecting.

I guess it’s time for me to have a look at my development environment and see if I can get something better.