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1+synopsis: Display capture hints in router layout
2+prs: #1556
3+
4+description: {
5+
6+This PR enhances the `Servant.Server.layout` function, which produces a textual description of the routing layout of an API. More precisely, it changes `<capture>` blocks, so that they display the name and type of the variable being captured instead.
7+
8+Example:
9+
10+For the following API
11+```haskell
12+type API =
13+ "a" :> "d" :> Get '[JSON] NoContent
14+ :<|> "b" :> Capture "x" Int :> Get '[JSON] Bool
15+ :<|> "a" :> "e" :> Get '[JSON] Int
16+```
17+
18+we previously got the following output:
19+
20+```
21+/
22+├─ a/
23+│ ├─ d/
24+│ │ └─•
25+│ └─ e/
26+│ └─•
27+└─ b/
28+ └─ <capture>/
29+ ├─•
30+ ┆
31+ └─•
32+```
33+
34+now we get:
35+
36+```
37+/
38+├─ a/
39+│ ├─ d/
40+│ │ └─•
41+│ └─ e/
42+│ └─•
43+└─ b/
44+ └─ <x::Int>/
45+ ├─•
46+ ┆
47+ └─•
48+```
49+
50+This change is achieved by the introduction of a CaptureHint type, which is passed as an extra argument to the CaptureRouter and CaptureAllRouter constructors for the Router' type.
51+CaptureHint values are then used in routerLayout, to display the name and type of captured values, instead of just `<capture>` previously.
52+
53+N.B.:
54+Because the choice smart constructor for routers can aggregate Capture combinators with different capture hints, the Capture*Router constructors actually take a list of CaptureHint, instead of a single one.
55+
56+This PR also introduces Spec tests for the routerLayout function.
57+
58+Warning:
59+This change is potentially breaking, because it adds the constraint `Typeable a` to all types that are to be captured. Because all types are typeable since GHC 7.10, this is not as bad as it sounds ; it only break expressions where `a` is quantified in an expression with `Capture a`.
60+In those cases, the fix is easy: it suffices to add `Typeable a` to the left-hand side of the quantification constraint.
61+
62+For instance, the following code will no longer compile:
63+```haskell
64+type MyAPI a = Capture "foo" a :> Get '[JSON] ()
65+
66+myServer :: forall a. Server (MyAPI a)
67+myServer = const $ return ()
68+
69+myApi :: forall a. Proxy (MyAPI a)
70+myApi = Proxy
71+
72+app :: forall a. (FromHttpApiData a) => Application
73+app = serve (myApi @a) (myServer @a)
74+```
75+
76+Indeed, `app` should be replaced with:
77+```haskell
78+app :: forall a. (FromHttpApiData a, Typeable a) => Application
79+app = serve (myApi @a) (myServer @a)
80+```
81+}