functional programming - How do I get keepWhen behaviour in Elm 0.15? -

the keepwhen function earlier versions of elm removed. have ported elm application 0.14, i'm stuck @ trying 1 part of work, it's using keepwhen.

so i'm looking function like

keepwhen : signal bool -> -> signal -> signal 

i have found

filter : (a -> bool) -> -> signal -> signal 

but it's not quite same thing, , haven't figured out how work.

answer: import utility package

the easiest way use signal.extra.keepwhen signal-extra package.
(full disclosure: i'm author)

important implementation detail

notice implementation isn't trivial. implementation in package (the signal module imported unqualified):

keepwhen : signal bool -> -> signal -> signal keepwhen boolsig asig =   zip boolsig asig   |> sampleon asig   |> keepif fst (true, a)   |> map snd 

the important difference version in kqr's answer sampleon keeps output of keepwhen updating when boolean input changes. difference between 2 filters keepwhen 0.14 filters update events a input , doesn't sample when boolean input becomes true.

the other implementation in signal-extra under name samplewhen.

diagrams

if know little marble diagrams, maybe these old diagrams may help. i'll post screenshot of relevant ones below.

the way read these diagrams:

1. time flows left right.
2. a line signal.
3. the block function takes 2 signals above , produces signal below.
4. the shape @ left of each line initial value.
5. a filled shape event on signal.
6. an outlined shape when signal doesn't change.
7. i've used shapes represent types.
8. i've used colours represent different values.

note second diagram, labeled old behaviour, matches behaviour of code in kqr's answer.