sml-middleware

Composable handler -> handler middleware for the pure SML HTTP core. A handler is request -> response (sml-http's model); middlewares wrap handlers and stack with ordinary composition. Everything is pure and deterministic — logs accumulate in a caller-owned sink and static files are served from an in-memory map, not the OS. Dual-compiler: MLton + Poly/ML.

Features

• compose — stack middlewares left-to-right (compose [a, b] h = a (b h)), first listed wraps outermost.
• catchErrors — turn a raising handler into a response (e.g. a 500).
• limitBody — reject oversized request bodies with 413 Payload Too Large.
• logTo — append a formatted log line per request to a string list ref.
• addHeader — set a response header on every response (e.g. security headers).
• static — serve GET/HEAD from an in-memory file map; keys match by exact path or sml-glob pattern, otherwise the request falls through to the inner handler.

API

type handler    = Http.request -> Http.response
type middleware = handler -> handler

val compose     : middleware list -> middleware
val catchErrors : (exn -> Http.response) -> middleware
val limitBody   : int -> middleware
val logTo       : string list ref -> (Http.request * Http.response -> string) -> middleware
val addHeader   : string -> string -> middleware
val static      : (string * (string * string)) list -> middleware

Example

val app =
  Middleware.compose
    [ Middleware.logTo sink fmt
    , Middleware.catchErrors (fn _ => Http.text 500 "Internal Error")
    , Middleware.addHeader "X-Powered-By" "sml-web"
    , Middleware.static [("/", ("text/html", "<h1>Welcome</h1>")),
                         ("/static/*", ("text/plain", "asset"))] ]
    (fn _ => Http.text 404 "Not Found")

val res = app someRequest

Run the bundled example (over hand-built requests):

make example

Build & test

make test        # MLton
make test-poly   # Poly/ML
make all-tests   # both

20 deterministic checks, identical under MLton and Poly/ML.

Installation

package github.com/sjqtentacles/sml-middleware
require {
  github.com/sjqtentacles/sml-http
  github.com/sjqtentacles/sml-glob
}

Dependencies are also vendored under lib/github.com/sjqtentacles/ and committed, so make needs no network.

Layout

lib/github.com/sjqtentacles/sml-middleware/
  middleware.sig  middleware.sml
  sources.mlb     sml-middleware.mlb
examples/app.sml                       stacked example app
test/                                  Harness suite (20 checks)

License

MIT