# Type Machine
TypeScript offers [*Utility Types*](https://www.typescriptlang.org/docs/handbook/utility-types.html), which allows creating a type from another.
There is no way of doing this in Haskell. You have to maintain all your types yourselves, and handle conversions from one to another yourself.
`type-machine` brings a solution to this problem. Using Template Haskell, generate new types using Type-Script-inspired functions like `omit`, `pick` and `record`.
It can also generate a conversion type-class that allows you to access fields and convert one type to another.
- Requirements
- Requires a couple of language extensions (see example)
- Input ADT must have exactly one record constructor
- Limitations
- Does not support type parameters yet
- `require` and `partial` only work with `Maybe` fields
## Examples
```haskell
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE DuplicateRecordFields #-}
data User = User {
id :: Int,
name :: String,
email :: Maybe String
}
$(type_ "UserWithEmail" (required ["email"] <::> ''User))
-- data UserWithEmail = UserWithEmail {
-- id :: Int,
-- name :: String,
-- email :: String
-- }
$(type_ "UserWithoutId" (omit ["id"] <::> ''User))
-- data UserWithoutId = UserWithoutId {
-- name :: String,
-- email :: String
-- }
$(type_ "UserId" (pick ["id"] <::> ''User))
-- data UserId = UserId {
-- id :: Int
-- }
$(type_ "Vector3" (record ["x", "y", "z"] [t|Int|]))
-- data Vector3 = Vector3 {
-- x :: Int,
-- y :: Int,
-- z :: Int
-- }
-----
-- Type Parameters
-----
data MyMaybe a = { content :: Maybe a }
$(type_ "MyString" (apply [t|String|] <::> ''MyMaybe))
-- data MyString = MyString {
-- content :: Maybe String
-- }
-----
-- Is
-----
$(declareIs ''User)
-- class IsUser a where
-- getId :: a -> Int
-- getName :: a -> String
-- getEmail :: a -> String
-- setId :: Int -> a -> a
-- setName :: String -> a -> a
-- setEmail :: String -> a -> a
--
-- instance IsUser User where
-- getId = id
-- getName = name
-- getEmail = email
-- setId = ...
-- setName = ...
-- setEmail = ...
$(type_ "UserWithoutEmail" (omit ["email"] <::> ''User))
$(deriveIs ''User ''UserWithoutEmail)
-- instance IsUser UserWithoutEmail where
-- ...
$(type_ "UserWithoutId" (omit ["id"] <::> ''User))
$(deriveIs ''User ''UserWithoutId) -- Will fail
```