[elm-discuss] Proposal: move Random.Extra.constant to Random

[Francesco Orsenigo]

Currently, the user has to either install `Random.Extra` just to use `
constant`, or define `constant a = Random.map (\_ -> a) Random.bool`, which
is a non-intuitive way to express the problem.

`constant` is the *return* operator of the Random monad, and as such
constitutes a very basic block for composing generators; it's shouldn't be
an "extra".
`Random` is the *only* core monad that doesn't have a return operator
within core.
Rather than a justification for having it in core, I think there should be
a justification for excluding it.
`Json.Decode.succeed` can be defined as `succeed x = Json.Decode.map (\_ ->
x) (Json.Decode.maybe Json.Decode.value)`, yet few would make a case that
it should not exist inside `Json.Decode`.

Implementing `constant` within `Random.Extra` currently forces a contrived
(and wasteful) workaround, generating a value just to throw it away:
https://github.com/elm-community/random-extra/blob/master/src/Random/Extra.elm#L33-L39

Moving `constant` to the `Random` would allow the implementation to become
trivial:
https://github.com/mgold/elm-random-pcg/blob/5.0.1/src/Random/Pcg.elm#L365


The absence from the core libraries also makes an implicit statement: that
most problems can be solved without it, and `constant` is needed only for
very complicated Generators.
While this might be true for very trivial Generators, it is not true as
soon as a Generator is required to produce anything non trivial:


type Shape
= Point
| Circle Int
| Rectangle Int Int

int : Generator Int
int =
Random.int 1 10

shapeGenerator : Generator Shape
shapeGenerator =
[ ( 0.3, Random.constant Point )
, ( 0.4, Random.map Circle int )
, ( 0.3, Random.map2 Rectangle int int )
]
|> Random.frequency


More in general, `constant` is necessary whenever a result could be,
depending on the arguments, deterministic or randomly generated: the same
function cannot return sometimes a value and sometimes a Generator.
Many such use cases are encountered in game development, and in the
#gamedev Slack channel we quickly collected several cases:

* Depending on the game state, enemy bot AI will sometime make
deterministic choices, sometimes random ones.

* Dungeon generation:
https://github.com/mordrax/cotwelm/blob/master/src/Dungeon/DungeonGenerator.elm#L98
https://github.com/mordrax/cotwelm/blob/master/src/Dungeon/DungeonGenerator.elm#L106
https://github.com/mordrax/cotwelm/blob/master/src/Dungeon/DungeonGenerator.elm#L159
https://github.com/mordrax/cotwelm/blob/master/src/Dungeon/DungeonGenerator.elm#L165
https://github.com/mordrax/cotwelm/blob/master/src/Dungeon/DungeonGenerator.elm#L167
https://github.com/mordrax/cotwelm/blob/master/src/Dungeon/DungeonGenerator.elm#L185
https://github.com/mordrax/cotwelm/blob/master/src/Dungeon/DungeonGenerator.elm#L190
https://github.com/mordrax/cotwelm/blob/master/src/Dungeon/DungeonGenerator.elm#L205
https://github.com/mordrax/cotwelm/blob/master/src/Dungeon/DungeonGenerator.elm#L241
https://github.com/mordrax/cotwelm/blob/master/src/Dungeon/DungeonGenerator.elm#L327

* Lexicon-based random word generation:
https://github.com/xarvh/lexical-random-generator/blob/1.0.0/src/LexicalRandom.elm#L95
https://github.com/xarvh/lexical-random-generator/blob/1.0.0/src/LexicalRandom.elm#L109
https://github.com/xarvh/lexical-random-generator/blob/1.0.0/src/LexicalRandom.elm#L117

[Martin Janiczek]

Thoughts, people?
I approve of this. `Random.constant` is a very basic building block for
defining Generators for me.

On Thursday, September 28, 2017 at 3:56:34 AM UTC+2, Francesco Orsenigo

Post by Francesco Orsenigo
Currently, the user has to either install `Random.Extra` just to use `
constant`, or define `constant a = Random.map (\_ -> a) Random.bool`,
which is a non-intuitive way to express the problem.
`constant` is the *return* operator of the Random monad, and as such
constitutes a very basic block for composing generators; it's shouldn't be
an "extra".
`Random` is the *only* core monad that doesn't have a return operator
within core.
Rather than a justification for having it in core, I think there should be
a justification for excluding it.
`Json.Decode.succeed` can be defined as `succeed x = Json.Decode.map (\_
-> x) (Json.Decode.maybe Json.Decode.value)`, yet few would make a case
that it should not exist inside `Json.Decode`.
Implementing `constant` within `Random.Extra` currently forces a
contrived (and wasteful) workaround, generating a value just to throw it
https://github.com/elm-community/random-extra/blob/master/src/Random/Extra.elm#L33-L39
Moving `constant` to the `Random` would allow the implementation to
https://github.com/mgold/elm-random-pcg/blob/5.0.1/src/Random/Pcg.elm#L365
The absence from the core libraries also makes an implicit statement: that
most problems can be solved without it, and `constant` is needed only for
very complicated Generators.
While this might be true for very trivial Generators, it is not true as
type Shape
= Point
| Circle Int
| Rectangle Int Int
int : Generator Int
int =
Random.int 1 10
shapeGenerator : Generator Shape
shapeGenerator =
[ ( 0.3, Random.constant Point )
, ( 0.4, Random.map Circle int )
, ( 0.3, Random.map2 Rectangle int int )
]
|> Random.frequency
More in general, `constant` is necessary whenever a result could be,
depending on the arguments, deterministic or randomly generated: the same
function cannot return sometimes a value and sometimes a Generator.
Many such use cases are encountered in game development, and in the
* Depending on the game state, enemy bot AI will sometime make
deterministic choices, sometimes random ones.
https://github.com/mordrax/cotwelm/blob/master/src/Dungeon/DungeonGenerator.elm#L98
https://github.com/mordrax/cotwelm/blob/master/src/Dungeon/DungeonGenerator.elm#L106
https://github.com/mordrax/cotwelm/blob/master/src/Dungeon/DungeonGenerator.elm#L159
https://github.com/mordrax/cotwelm/blob/master/src/Dungeon/DungeonGenerator.elm#L165
https://github.com/mordrax/cotwelm/blob/master/src/Dungeon/DungeonGenerator.elm#L167
https://github.com/mordrax/cotwelm/blob/master/src/Dungeon/DungeonGenerator.elm#L185
https://github.com/mordrax/cotwelm/blob/master/src/Dungeon/DungeonGenerator.elm#L190
https://github.com/mordrax/cotwelm/blob/master/src/Dungeon/DungeonGenerator.elm#L205
https://github.com/mordrax/cotwelm/blob/master/src/Dungeon/DungeonGenerator.elm#L241
https://github.com/mordrax/cotwelm/blob/master/src/Dungeon/DungeonGenerator.elm#L327
https://github.com/xarvh/lexical-random-generator/blob/1.0.0/src/LexicalRandom.elm#L95
https://github.com/xarvh/lexical-random-generator/blob/1.0.0/src/LexicalRandom.elm#L109
https://github.com/xarvh/lexical-random-generator/blob/1.0.0/src/LexicalRandom.elm#L117

[Max Goldstein]

I also think it’s a good idea. I’ve brought it up with Evan and Richard before. But I can tell you from experience that getting changes merged into the Random module is very difficult.

Also, shameless plug for mgold/elm-random-pcg, which includes this and many other useful functions built on a superior PRNG.

[Martin Janiczek]

IIRC from conversations with Francesco, this thread started as a reaction to seeing that core's (to be 0.19) Random incorporated your Random.PCG's code but omitted `constant`.

Can we know the reasoning behind the "it shouldn't be there" opinion?