# Exceptions vs. Return Values to represent errors (in F#) – IV – Implementation

## 2012/12/07

Tags: F#

The Critical monad is defined as follows. First there is the type that propagates through the monad:

```type Result<'a, 'b> =
| Success of 'a
| Failure of 'b```

Then we define the usual computation expression methods.

```type Critical() =
// a -> m a
member o.Return x       = Success x
// m a -> (a -> m b) -> m b
member o.Bind (m, f)    = match m with
| Failure e -> Failure e
| Success x -> f x
// m a -> m a
member o.ReturnFrom m   = m```

Explaining how computational expressions work in F# is a blog onto itself. And several chapters in many books. Sufficient to say that conceptually this propagates the success value and returns the failure value.

We then define an instance of this type, so that we can use the nice ‘critical { … }’ syntax.

```let critical = Critical()
```

We then go and define the functions that the user needs to use to annotate their function calls. The simplest one is the one to propagate any exception coming from the function ‘f’.

```let fault f = f
```

Then it comes the one to manage contingencies. This will trap any exception for which ‘stopF ex’ is ‘true’, call ‘errF ex’ to construct the error return value and wrap it in a ‘Failure’. Otherwise it will rethrow the exception.

```let contingentGen stopF errF f =
try
Success(f ())
with
| ex when stopF ex -> Failure(errF ex)
| _                -> reraise ()
```

Albeit very simple, the above is the core of the system. Everything else is just details. Let’s look at them.

First we want a function that takes as parameter a list of (Exception, ReturnValue) and gives back the correct stopF errF to plug into ‘contingentGen’.

```let exceptionMapToFuncs exMap =
let tryFind ex = exMap |> List.tryFind (fun (k, _) -> k.GetType() = ex.GetType())
(fun ex ->
let found = tryFind ex
match found with Some(_) -> true | None -> false),
(fun ex ->
let found = tryFind ex
match found with
| Some(k, v)    -> v ex
| None          -> raise ex)
```

Then ugliness comes. For the sake of getting a decent syntax (not great) on the calling site, we need to fake overloading of functions by the old trick of adding a number at the end. Thanks to Tobias to point out this (my api was even worse earlier).

```let contingent1 exMap f x =
let stopF, errF = exceptionMapToFuncs exMap
contingentGen stopF errF (fun _ -> f x)
let contingent2 exMap f x y =
let stopF, errF = exceptionMapToFuncs exMap
contingentGen stopF errF (fun _ -> f x y)
let contingent3 exMap f x y z =
let stopF, errF = exceptionMapToFuncs exMap
contingentGen stopF errF (fun _ -> f x y z)
```

Sometimes you want to trap all exceptions from a function and return your own error value:

```let neverThrow1 exc f x     = contingentGen (fun _ -> true) (fun ex -> exc ex) (fun _ -> f x)
let neverThrow2 exc f x y   = contingentGen (fun _ -> true) (fun ex -> exc ex) (fun _ -> f x y)
let neverThrow3 exc f x y z = contingentGen (fun _ -> true) (fun ex -> exc ex) (fun _ -> f x y z)
```

Other times you need to go from a function that returns return values to one that throws exceptions. You need translating from contingencies to faults:

```let alwaysThrow exc f x =
match f x with
| Success(ret)              -> ret
| Failure(e)                -> raise (exc e)
```

And that’s it. Hopefully we have bridged the gap between exceptions and return values without making the code too ugly (just a little bit). Or perhaps not.

I need to add that I haven’t used this library myself (yet). I’m sure when I do I’ll discover many things to change.