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I'm trying to wrap my head around FRP and I'm not sure that I'm doing it right. I want to build up a string from key press events up until enter is pressed. Once enter is pressed, the string gets written out and the accumulator is reset to the empty string.

I have a event source that emits a Char every time a key on the keyboard is pressed, ePressed. First, I separate out the two kinds of key presses I care about:

eWritable = filterE (`elem` ['A'..'z']) ePressed
eEnter = filterE (== '\n') ePressed

Now I know how to gather them together into what I want to send out:

eToPrint = accumE "" (fmap (:) eWritable)

But I'm not sure how to "hold on to" this until enter is pressed, or how to reset it afterwards. What's the right, idomatic way to go about this?

Patrick Collins
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1 Answers1

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The idea is that eToPrint is sort of a union of two events: when you press characters and when you press enter. Here an example (reactive-banana 0.8):

eToPrint = accumE "" $ unions [(:) <$> eWritable, const "" <$> eEnter]

To "hold" it, you can use a Behavior.

Here is a complete solution:

bString = accumB "" $ unions [(:) <$> eWritable, const "" <$> eEnter]
eOut    = bString <@ eEnter

The behavior bString contains the accumulated String value. The eOut event returns the last string value whenever the eEnter event happens. Note in particular the semantics of accumB: At the moment in time where eEnter happens, the value of bString is still the old value.

Heinrich Apfelmus
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