https://github.com/eeue56/meta-elm
Examples of doing meta programming Elm, which you shouldn't do
https://github.com/eeue56/meta-elm
Last synced: 18 days ago
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Examples of doing meta programming Elm, which you shouldn't do
- Host: GitHub
- URL: https://github.com/eeue56/meta-elm
- Owner: eeue56
- License: bsd-3-clause
- Created: 2015-11-02T14:26:40.000Z (over 9 years ago)
- Default Branch: master
- Last Pushed: 2016-02-27T11:08:40.000Z (about 9 years ago)
- Last Synced: 2025-04-15T01:14:51.812Z (18 days ago)
- Language: Elm
- Homepage:
- Size: 11.7 KB
- Stars: 7
- Watchers: 2
- Forks: 2
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
- License: LICENSE
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README
# meta-elm
Welcome to meta-elm! In here, we will look at examples of how to abuse Elm to make it work for meta-programming. Note the word _abuse_. Don't do any of this for anything other than research or experiments!
While some of these implementations are type-safe and correct, others are not. This means that it's not a good thing to add to any production Elm-based systems, as it removes one of the major benefits of having a static strongly-typed language.
For example, a implementation of fmap:
```elm
age =
Just 5
-- equal to 6
newAge =
Meta.fmap (\x -> x + 1) age
-- equal to Nothing
newerAge =
Meta.fmap (\x -> x + 1) Nothing
type Age = Years Int | Days Int
-- equal to Years 10
newNewAge =
Meta.fmap (\x -> x + 5) <| Years 5
```
Or how about changing the type of an object at runtime:
```elm
type Animal = Cat | Dog
type Fish = Fish (Maybe Int)
myCat =
Cat
-- equal to Dog
myCatIsADog =
Meta.switchType myCat Dog
-- equal to "Yup, he's a dog"
isMyCatReallyADog =
case myCatIsADog of
Dog -> "Yup, he's a dog!"
Cat -> "Nope, he's a cat."
_ -> "IDK what he is."
-- equal to Fish _
myDogCatIsNowAFish =
Meta.switchType myCatIsADog (Fish (Just 1))
-- equal to "Yup, fishy!"
isMyDogCatReallyAFish =
case myDogCatIsNowAFish of
Fish _ -> "Yup, fishy!"
_ -> "Nope, something else"
```
Use getters for self-referential properties
```elm
type alias Person = {
age : Int,
name : String,
realAge : Int,
actualAge : Meta.Getter Int
}
-- need to provide a default value for the getter
me = {
name = "Dave",
age = 12,
realAge = 21,
actualAge = Meta.Getter -1 }
-- create a getter on the me object
me' =
Meta.getter
(\me -> if me.age < 20 then me.realAge else me.age)
me.actualAge
me
-- equal to 21
myAge =
me'.actualAge
```