ttc-1.1.1.1: README.md
# TTC: Textual Type Classes
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* [Overview](#overview)
* [`Textual`](#textual)
* [`Render`](#render)
* [`Parse`](#parse)
* [Constant Validation](#constant-validation)
* [Related Work](#related-work)
* [Rendering and Parsing](#rendering-and-parsing)
* [Validating Constants](#validating-constants)
* [String Type Conversion](#string-type-conversion)
* [Arbitrary Type Conversion](#arbitrary-type-conversion)
* [Project](#project)
* [Links](#links)
* [Tags](#tags)
* [Contribution](#contribution)
* [License](#license)
## Overview
TTC, an initialism of _Textual Type Classes_, is a library that provides
`Render` and `Parse` type classes for conversion between data types and
textual data types (strings). Use the `Show` and `Read` type classes for
debugging/development, and use the `Render` and `Parse` type classes for your
own purposes. The library also provides a `Textual` type class for conversion
between textual data types as well as functions for validating constants at
compile-time.
Since a type may have at most one instance of a given type class, special care
must be taken when defining type class instances in a shared library. In
particular, orphan instances should generally *not* be used in shared
libraries since they prevent users of the libraries from writing their own
instances.
`Render` and `Parse` are best used with types that have canonical textual
representations, such as textual identifiers. When there is more than one way
to create a textual representation, such as configurable formatting, using a
normal function is probably more appropriate. Such a function can make use of
the `Textual` type class to support multiple textual data types.
This overview includes a brief introduction of the library. The following
resources are also available:
* [API documentation][] is viewable on Hackage.
* A [series of articles][] gives a guided tour of the library.
1. [Textual Type Class][]
2. [Render and Parse][]
3. [Validated Constants][]
4. [Best Practices][]
* The [examples][] directory in the GitHub repository contains usage examples.
[API documentation]: <https://hackage.haskell.org/package/ttc#modules>
[series of articles]: <https://www.extrema.is/articles/ttc-textual-type-classes>
[Textual Type Class]: <https://www.extrema.is/articles/ttc-textual-type-classes/textual-type-class>
[Render and Parse]: <https://www.extrema.is/articles/ttc-textual-type-classes/render-and-parse>
[Validated Constants]: <https://www.extrema.is/articles/ttc-textual-type-classes/validated-constants>
[Best Practices]: <https://www.extrema.is/articles/ttc-textual-type-classes/best-practices>
[examples]: <https://github.com/ExtremaIS/ttc-haskell/tree/main/examples>
### `Textual`
The `Textual` type class is used to convert between the following textual data
types:
* `String`
* Strict `Text`
* Lazy `Text`
* `Text` `Builder`
* Strict `ByteString`
* Lazy `ByteString`
* `ByteString` `Builder` (and `Data.Binary.Builder`)
* `ShortByteString`
This type class has two key features:
* Type conversion is *not* done through a fixed type (such as `String` or
`Text`).
* It has a single type variable, making it easy to write functions that
accept arguments and/or return values that may be any of the supported
textual data types.
For more details, see the [Textual Type Class][] article.
### `Render`
The `Render` type class renders a data type as a `Textual` data type:
```haskell
class Render a where
render :: Textual t => a -> t
```
It is analogous to the `Show` type class, which can be reserved for
debugging/development.
The `render` function returns any of the supported textual data types. Use
the textual data type that is most natural in the implementation of `render`
instances, and return values are converted to other textual data types when
necessary. The `Show` and `IsString` type classes are not used, so use of the
`String` type is not required.
As a simple example, consider a `Username` type that is implemented as a
`newtype` over `Text`:
```haskell
module Username (Username) where
import Control.Monad (unless, when)
import Data.Char (isAsciiLower)
import qualified Data.Text as T
import Data.Text (Text)
import qualified Data.TTC as TTC
newtype Username = Username Text
deriving (Eq, Ord, Show)
instance TTC.Render Username where
render (Username t) = TTC.convert t
```
If a username needs to be included in a `String` error message, conversion is
automatic:
```haskell
putStrLn $ "user not found: " ++ TTC.render uname
```
For more details, see the [Render and Parse][] article.
### `Parse`
The `Parse` type class parses a data type from a `Textual` data type:
```haskell
class Parse a where
parse :: (Textual t, Textual e) => t -> Either e a
```
It is analogous to the `Read` type class, which can be reserved for
debugging/development.
The `parse` function takes any of the supported textual data types as an
argument. Use the textual data type that is most natural in the
implementation of `parse` instances, and arguments are converted from other
textual data types when necessary. The `IsString` type class is not used, so
use of the `String` type is not required.
Here is an example instance for `Username`, implementing some restrictions:
```haskell
instance TTC.Parse Username where
parse = TTC.asT $ \t -> first TTC.fromS $ do
unless (T.all isAsciiLower t) $ Left "username has invalid character(s)"
let len = T.length t
when (len < 3) $ Left "username has fewer than 3 characters"
when (len > 12) $ Left "username has more than 12 characters"
pure $ Username t
```
If a username needs to be parsed from a `String`, conversion is automatic:
```haskell
case TTC.parse "tcard" :: Either String Username of
Right uname -> putStrLn $ "valid username: " ++ TTC.render uname
Left err -> putStrLn $ "invalid username: " ++ err
```
For more details, see the [Render and Parse][] article.
### Constant Validation
TTC provides functions to validate constants at compile-time, using Template
Haskell. For example, a `Username` constant can be defined as follows:
```haskell
user :: Username
user = $$(TTC.valid "tcard")
```
For more details, see the [Validated Constants][] article.
## Related Work
### Rendering and Parsing
The [relude](https://hackage.haskell.org/package/relude) library has
polymorphic versions of `show` and `readEither` in `Relude.String.Conversion`,
as well as various type classes for converting between string types. This
does not encourage using `Show` and `Read` instances with syntactically valid
Haskell syntax, and it encourages the using of the `String` data type.
The [rio](https://hackage.haskell.org/package/rio) library has a `Display`
type class with a similar goal as `TTC.Render`. Since the library encourages
a uniform usage of textual data types, `Display` only provides functions for
rendering to `Text` and a builder format. It does not have a type class
similar to `TTC.Parse`.
The [text-display](https://hackage.haskell.org/package/text-display) library
defines a `Display` type class intended to render user-facing text. It uses a
`Builder` type internally and renders to a `Text` value.
Harry Garrood has an interesting series of blog posts about type classes and
`Show`:
* [Down with Show! Part 1: Rules of thumb for when to use a type class](https://harry.garrood.me/blog/down-with-show-part-1/)
* [Down with Show! Part 2: What's wrong with the Show type class](https://harry.garrood.me/blog/down-with-show-part-2/)
* [Down with Show! Part 3: A replacement for Show](https://harry.garrood.me/blog/down-with-show-part-3/)
### Validating Constants
The [qq-literals](https://hackage.haskell.org/package/qq-literals) library
creates a `QuasiQuoter` from a parse function of type
`String -> Either String a`. The functionality is similar to TTC's
`mkUntypedValidQQ` function. The `mkUntypedValidQQ` function allows the user
to choose the name of the `QuasiQuoter` because a name like `valid` is
preferred when used via a qualified import while a name like `username` may be
preferred when not using qualified imports. Note that `mkUntypedValidQQ` also
splices in an explicit type signature.
The
[validated-literals](https://hackage.haskell.org/package/validated-literals)
library has a `Validate` type class that is similar to `TTC.Parse` but
supports conversion between arbitrary types, not just from textual data types.
Template Haskell functions are provided to perform validation at compile-time.
Result types must either have `Lift` instances or equivalent implementations.
Chris Done posted
[a gist](https://gist.github.com/chrisdone/809296b769ee36d352ae4f8dbe89a364)
about implementing statically checked overloaded strings.
### String Type Conversion
There are a number of libraries that simplify conversion between string types.
The following libraries provide type classes with two type variables. The
primary benefit of this approach is that one can add support for any string
type. The drawback of this approach is that implementations of `Render` and
`Parse` using such a type class would have to be done via a fixed type,
resulting in unnecessary conversion when using other types.
* [string-conv](https://hackage.haskell.org/package/string-conv)
* [string-conversions](https://hackage.haskell.org/package/string-conversions)
The following library provide type classes with a single type variable, but
conversion is done via a fixed type.
* [hxt-regex-xmlschema](https://hackage.haskell.org/package/hxt-regex-xmlschema)
has a `StringLike` type class and does conversion via the `String` type
* [ListLike](https://hackage.haskell.org/package/ListLike) has a `StringLike`
type class and does conversion via the `String` type
* [monoid-subclasses](https://hackage.haskell.org/package/monoid-subclasses)
provides a `TextualMonoid` type class that provides an abstract API over
textual types, using `String` as the underlying type
* [stringlike](https://hackage.haskell.org/package/stringlike) converts via
the `Text` type
* [tagsoup](https://hackage.haskell.org/package/tagsoup) has a `StringLike`
type class that provides an abstract API over textual types and a
`castString` function that converts via the `String` type
* [text-conversions](https://hackage.haskell.org/package/text-conversions)
converts via the `Text` type
* [textual](https://hackage.haskell.org/package/textual) (deprecated)
converts via the `String` type
### Arbitrary Type Conversion
There are also a number of libraries that provide type classes for conversion
between arbitrary types, including string types.
* [basement](https://hackage.haskell.org/package/basement) provides type
classes for conversion that may fail as well as conversion that cannot fail
* [convertible](https://hackage.haskell.org/package/convertible)
* [witch](https://hackage.haskell.org/package/witch) provides type classes for
conversion that may fail as well as conversion that cannot fail
## Project
### Links
* Hackage: <https://hackage.haskell.org/package/ttc>
* Stackage: <https://stackage.org/package/ttc>
* GitHub: <https://github.com/ExtremaIS/ttc-haskell>
* GitHub Actions CI: <https://github.com/ExtremaIS/ttc-haskell/actions>
### Tags
All releases are tagged in the `main` branch. Release tags are signed using
the
[`security@extrema.is` GPG key](http://keys.gnupg.net/pks/lookup?op=vindex&fingerprint=on&search=0x1D484E4B4705FADF).
### Contribution
Issues and feature requests are tracked on GitHub:
<https://github.com/ExtremaIS/ttc-haskell/issues>
Issues may also be submitted via email to <bugs@extrema.is>.
### License
This project is released under the
[MIT License](https://opensource.org/licenses/MIT) as specified in the
[`LICENSE`](LICENSE) file.