73 lines
2.2 KiB
Markdown
73 lines
2.2 KiB
Markdown
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# Cow Description Language (CDL)
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In times of the pandemic, it's not easy to get real input data from your cows to
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demonstrate the functionality of your front end. To be able to test some
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features of the application, we would like to have scripts that
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insert fake data points into our database, allowing us how our app
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would behave in real time, without the need to hook up any collars.
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These scripts would be tedius to write...
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Unless, of course, you have a Domain Specific Language (DSL) for it!
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Enter CDL. We can describe a cow in a turtle-style imperative language:
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```Haskell
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squareCow = do
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move 10
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turn 90
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move 10
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turn 90
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move 10
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turn 90
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move 10
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```
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This DSL is shallowly embedded into Haskell, which means that we can
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use all of Haskell's abstraction capabilities:
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```Haskell
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repeat 0 _ = return ()
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repeat n thing = do
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thing
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repeat (n-1) thing
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squareCow = repeat 4 $ do
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move 10
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turn 90
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```
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Given a sampling frequency, CDL can generate a Python script that
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inserts data points into our database each "sample":
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```Python
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from server.fgs.model import *
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from server.fgs import db
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import datetime
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db.session.add(DataPoint(longitude=0.0, latitude=0.0, datetime=datetime.now()))
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db.session.commit()
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sleep(0.1)
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db.session.add(DataPoint(longitude=9.0000094e-7, latitude=0.0, datetime=datetime.now()))
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db.session.commit()
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sleep(0.1)
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db.session.add(DataPoint(longitude=1.8000019e-6, latitude=0.0, datetime=datetime.now()))
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db.session.commit()
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sleep(0.1)
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# ...
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```
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To generate scripts, load the program into GHCI and execute as follows:
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```
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cd src
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ghci
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:load Cdl/SampleCows.hs Cdl/Codegen.hs Cdl/Cow.hs
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:module Cdl.SampleCows Cdl.Codegen Cdl.Cow
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outputCow "test.py" 0.1 (1, 1, Coord 0 0, squareCow)
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```
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In `outputCow`, the `0.1` is the sampling frequency (how often
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the database will receive mock data from the cows), the first `1` is
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the collar ID sending the data, the second `1` is the cow's walking speed (in meters/second),
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the `Coord 0 0` is the initial longitude and latitude of the cow, and `squareCow` is
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the cow's actions described above.
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Note that the script relies on being in the same directory as the `server` repository
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containing our database model.
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