Hello elm folks,
I have a beginner question about Random.list:
Without access to the Generator type constructor, how can one build totally
novel kinds of list-based generators? For example, suppose you want to
construct a list generator that terminates not after an integer number of
iterations (i.e. Random.list), but after a condition determined by
inspecting one or more items in the list?
Example: *Generate a list of integers between 0 and 10 until the last one
is 5*.
It seems like this class of generators requires a version of Random.list
that accepts a function in place of the integer argument, which is passed
say, the last generated element of the list and returns a Bool indicating
whether the list is complete. It could look something like this:
list : (Maybe a -> Bool) -> Generator a -> Generator (List a)
list pred (Generator generate) =
Generator <| \seed ->
listHelp [] pred generate seed
listHelp : List a -> (Maybe a -> Bool) -> (Seed -> (a,Seed)) -> Seed -> (
List a, Seed)
listHelp list pred generate seed =
if pred <| head list then
(List.reverse list, seed)
else
let
(value, newSeed) =
generate seed
in
listHelp (value :: list) pred generate newSeed
Is it possible to construct this generator using only the primitives
currently exposed by Random? And if it can be done, why isn’t Random.list
itself implemented using those same primitives?
Thanks,
Matt
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