Okay, what I really wanted to do is, I have an Array and I want to choose a random element from it. The obvious thing to do is get an integer from a random number generator between 0 and the length minus 1, which I have working already, and then applying Array.get, but that returns a Maybe a
. (It appears there's also a package function that does the same thing.) Coming from Haskell, I get the type significance that it's protecting me from the case where my index was out of range, but I have control over the index and don't expect that to happen, so I'd just like to assume I got a Just
something and somewhat forcibly convert to a
. In Haskell this would be fromJust
or, if I was feeling verbose, fromMaybe (error "some message")
. How should I do this in Elm?
I found a discussion on the mailing list that seems to be discussing this, but it's been a while and I don't see the function I want in the standard library where the discussion suggests it would be.
Here are some pretty unsatisfying potential solutions I found so far:
- Just use withDefault. I do have a default value of
a
available, but I don't like this as it gives the completely wrong meaning to my code and will probably make debugging harder down the road. - Do some fiddling with ports to interface with Javascript and get an exception thrown there if it's Nothing. I haven't carefully investigated how this works yet, but apparently it's possible. But this just seems to mix up too many dependencies for what would otherwise be simple pure Elm.