How can I compare random number in elm

Question

I use float : Signal a -> Signal Float to generate random number.

I want to compare this generated number with a Float, how can I do that?

main = lift asText ((Random.float (fps 24)) < 0.3)

This is wrong, error message:

    (Random.float (fps 24)) < 0.3

    Expected Type: Bool
    Actual Type: Signal.Signal a

Answer 1

The best way to debug this type of thing is to follow the types. Break every part into a separate piece, examine the types and resolve the type error.

We have a few things here:

lift : (a -> b) -> Signal a -> Signal b
asText : a -> Element
Random.float : Signal a -> Signal Float
fps : number -> Signal Time
(<) : number -> number -> Bool

Okay, so let's start plugigng in values and resolving the types:

fps 24 : Signal Time
Random.float (fps 24) : Signal Float
(Random.float (fps 24)) < 0.3 : ???

This is where our type error is coming from. We are trying to pass in a Signal but the (<) operator only accepts number. So, what we actually want is a function that compares the value in our Signal to see if it is < 0.3.

foo : Float -> Bool
foo n = n < 0.3

Now, we can lift foo and pass the signal into it:

lift foo (Random.float (fps 24))

And then plug it back into our original expression:

main = list asText (lift foo (Random.float (fps 24)))

Now, all of the types resolve.

Hope this helps!