F#: warning FS0020: This expression should have type 'unit', but has type 'bool'

Question

I'm trying to learn F# by going through some of the Euler problems and I found an issue I haven't been able to figure out. This is my naive solution. ``` let compute = let mutable f = false let mutable nr = 0 while f = false do nr <- nr + 20 f = checkMod nr nr ``` When i do this I get the error message warning FS0020: This expression should have type 'unit', but has type 'bool' on the expression "nr <- nr +20". I've tried rewriting and moving the expressions around and I always get that error on the line below the while statement. I'm writing this using VS2010 Beta.

Accepted Answer

The following line:

```
f = checkMod nr
```

is an equality check, not an assignment as I believe you are intending. Change it to:

```
f <- checkMod nr
```

and all should work fine. I'm not sure why you've used the correct syntax on the previous line and not that line...

Also, the line `while f = false do` should really be simplified to `while not f do`; equality checks on booleans are rather convoluted.

As I side note, I feel a need to point out that you are effectively trying to use F# as an imperative language. Use of mutable variables and while loops are strongly discouraged in functional languages (including F#), especially when a purely functional (and simpler) solution exists, as in this situation. I recommend you read up a bit on programming in the functional style. Of course, just getting to grips with the syntax is a useful thing in itself.

F#: warning FS0020: This expression should have type 'unit', but has type 'bool'

Solution

Problem