Reverse a N-N mapping of data in Dhall
Given is a config of apps and containers with an app being able to have multiple containers a container being able to have multiple apps. I want to be able to output them in 2 ways
- Per app list the containers
- Per container list the apps
The data format is simple, but I can't seem to find a way to get both of these representations without repeating the relation.
Example data when starting from containers having apps
let app1 = { name = "app1" }
let app2 = { name = "app2" }
let containers = [
{ name = "container1", apps = [ app1 ] },
{ name = "container2", apps = [ app1, app2 ] }
]
{- I can easily transform this data to the following -}
[
{ app = "app1", container = "container1" },
{ app = "app1", container = "container2" },
{ app = "app2", container = "container2" }
]
{- But I cannot seem to get it into the requested format -}
[
"app1" = [ "container1", "container2" ]
"app2" = [ "container2" ]
]
I think using identifiers as Text
cannot work as there is no way to merge associated lists or something alike using equal identifiers.
Using records I can merge something like this {a1 = {c1 = True}} /\ {a1 = {c2 = True}} /\ {a2 = {c2 = True}}
. Which would be {a1 = {c1 = True, c2 = True}, a2 = {c2 = True}}
.
But I can't get to this state in the first place because I can't 'reverse' records.
I don't care how I need to structure the config as long as I don't need to repeat the relation twice.
Yeah, it's not possible to do exactly what you're requesting because the language does not permit Text
comparisons
The closest solution I can think of is something like this:
let Map = https://prelude.dhall-lang.org/v21.1.0/Map/Type.dhall
let List/concatMap = https://prelude.dhall-lang.org/v21.1.0/List/concatMap.dhall
let List/map = https://prelude.dhall-lang.org/v21.1.0/List/map.dhall
let startingMapping
: Map Text (Map Text {})
= toMap
{ container1 = toMap { app1 = {=} }
, container2 = toMap { app1 = {=}, app2 = {=} }
}
let desiredMapping
: Map Text (Map Text {})
= toMap
{ app1 = toMap { container1 = {=}, container2 = {=} }
, app2 = toMap { container2 = {=} }
}
let transpose
: ∀(a : Type) → Map Text (Map Text a) → Map Text (Map Text a)
= λ(a : Type) →
List/concatMap
{ mapKey : Text, mapValue : Map Text a }
{ mapKey : Text, mapValue : Map Text a }
( λ(x : { mapKey : Text, mapValue : Map Text a }) →
List/map
{ mapKey : Text, mapValue : a }
{ mapKey : Text, mapValue : Map Text a }
( λ(y : { mapKey : Text, mapValue : a }) →
{ mapKey = y.mapKey
, mapValue =
[ { mapKey = x.mapKey, mapValue = y.mapValue } ]
}
)
x.mapValue
)
in assert : transpose {} startingMapping ≡ desiredMapping
The assertion fails, though, with this error message:
Error: Assertion failed
[ - { mapKey = "app1"
, mapValue = [ { mapKey = "container1", mapValue = {=} } ]
}
, - { mapKey = "app1"
, mapValue = [ { mapKey = "container2", mapValue = {=} } ]
}
, + { mapKey = "app1"
, mapValue =
[ { mapKey = "container1", mapValue = {=} }
, { mapKey = "container2", mapValue = {=} }
]
}
, …
]
41│ assert : transpose {} startingMapping ≡ desiredMapping
… because the result does not consolidate the two duplicate app1
keys like you were requesting.