Can create a crash by returning an invalid fuzzer from `andThen`

[jwoudenberg]

This fuzzer will run into a Debug.crash:

myFuzz : Fuzzer Int
myFuzz =
    int |> andThen (\_ -> frequency [])

Error message reads:

    This test failed because it threw an exception: "Error: Ran into a `Debug.crash` in module `Fuzz.Internal` on line 63
    The message provided by the code author is:

        This shouldn't happen: Fuzz.Internal.unpackGenVal"

Looking at the code I don't think there's a ton we can do about this, but I wanted to report anyway. andThen, by its nature, ensures we only know whether a fuzzer is valid after running it which conflicts with the idea of fuzzer validness itself.

[jwoudenberg]

Found another crashing fuzzer in this category. This one is unrelated to andThen and seems fixable.

myFuzz : Fuzzer Int
myFuzz =
    frequency
        [ ( 1, frequency [] )
        ]

[mgold]

Is this fixed by #183?

[jwoudenberg]

@mgold Yes!

[mgold]

Current behavior:

↓ Example
✗ #160

    This test failed because it threw an exception: "Error: Ran into a `Debug.crash` in module `Fuzz.Internal`
    
    This was caused by the `case` expression between lines 54 and 59.
    One of the branches ended with a crash and the following value got through:
    
        Nothing
    
    The message provided by the code author is:
    
        Returning an invalid fuzzer from `andThen` is currently unsupported"