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1 | # blazeT [![Build Status](https://travis-ci.org/johannesgerer/blazeT.svg?branch=master)](https://travis-ci.org/johannesgerer/blazeT) [![Hackage](https://img.shields.io/hackage/v/blazeT.svg)](https://hackage.haskell.org/package/blazeT) | ||
2 | |||
3 | A true monad (transformer) version of the | ||
4 | [blaze-markup](https://hackage.haskell.org/package/blaze-markup) and | ||
5 | [blaze-html](https://hackage.haskell.org/package/blaze-html) | ||
6 | libraries. | ||
7 | |||
8 | # Why? | ||
9 | |||
10 | While blaze's `Markup` and `Html` types have `Monad` instances and can | ||
11 | leverage the the concise `do` notation, they do not satisfy | ||
12 | the | ||
13 | [Monad Laws](https://hackage.haskell.org/package/base-4.8.0.0/docs/Control-Monad.html#t:Monad) and | ||
14 | thus cannot be used as Monads, let alone Monad transformers. | ||
15 | |||
16 | ## Use Cases | ||
17 | |||
18 | The `MarkupT` Monad Transformer enables us to write Markup (e.g. HTML) | ||
19 | templates that have access to all those Monads you cannot live without | ||
20 | anymore. | ||
21 | |||
22 | Accessing an environment | ||
23 | ([MonadReader](https://hackage.haskell.org/package/mtl-2.2.1/docs/Control-Monad-Reader-Class.html)), | ||
24 | accumulating log or other diagnostic output | ||
25 | ([MonadWriter](https://hackage.haskell.org/package/mtl-2.2.1/docs/Control-Monad-Writer-Class.html)), | ||
26 | doing `IO` (like database access) are the first things that come to | ||
27 | mind. | ||
28 | |||
29 | The initial reason of existence of this library is its use | ||
30 | in [Lykah](http://johannesgerer.com/Lykah), which powers my personal | ||
31 | website | ||
32 | [http://johannesgerer.com](http://johannesgerer.com/johannesgerer.com). In | ||
33 | Lykah, the HTML templates have access to the whole site structure (to | ||
34 | build things like menus) and automatically check, insert and keep | ||
35 | track of referenced pages and assets, which turns out to be very | ||
36 | useful for the task of static website generation. | ||
37 | |||
38 | # How to use it? | ||
39 | |||
40 | ## Backwards compatible | ||
41 | |||
42 | The library is intended to serve as a drop-in replacement for the | ||
43 | `blaze-markup` and `blaze-html` libraries and should be backwards | ||
44 | compatible: | ||
45 | |||
46 | Simply replace your `module Text.Blaze.*` imports with `module | ||
47 | Text.BlazeT.*` and it should give the same results. | ||
48 | |||
49 | For usage of blaze check out | ||
50 | their [documentation](https://jaspervdj.be/blaze/). | ||
51 | |||
52 | ## Unleash the monads | ||
53 | |||
54 | |||
55 | |||
56 | # Implementation | ||
57 | |||
58 | ... is located | ||
59 | in | ||
60 | [Text.BlazeT.Internals](https://hackage.haskell.org/package/blazeT/docs/Text-BlazeT-Internals.html). | ||
61 | |||
62 | Everything build around the simple `newtype` definition of the | ||
63 | `MarkupT` transformer, which makes use of the fact that `Blaze.Markup` | ||
64 | is | ||
65 | a | ||
66 | [Monoid](https://hackage.haskell.org/package/base-4.7.0.2/docs/Data-Monoid.html) and | ||
67 | which is basically a `WriterT` transformer writing `Blaze.Markup`: | ||
68 | |||
69 | ```Haskell | ||
70 | newtype MarkupT m a = MarkupT { fromMarkupT :: WriterT B.Markup m a } | ||
71 | ``` | ||
72 | |||
73 | Wrappers used to lift all `Blaze` entities into `BlazeT` are trivially | ||
74 | expressible using basic `WriterT` class methods. Wrapping `Blaze.Markup` is simply `WriterT.tell`: | ||
75 | |||
76 | ```Haskell | ||
77 | wrapMarkupT :: Monad m => B.Markup -> MarkupT m () | ||
78 | wrapMarkupT = tell | ||
79 | ``` | ||
80 | Wrapping functions that modify `Blaze.Markup` is simply `WriterT.censor`: | ||
81 | |||
82 | ```Haskell | ||
83 | wrapMarkupT2 :: Monad m => (B.Markup -> B.Markup) -> MarkupT m a -> MarkupT m a | ||
84 | wrapMarkupT2 = censor | ||
85 | ``` | ||
86 | |||
87 | |||