config: use new form + add example [skip ci]

* also unstage config.dhall and add it to the list of the ignored

.gitignore

@@ -1,6 +1,9 @@
 # main binary
 pcmt
 
+# dev config.
+/config.dhall
+
 # air tmp dir
 /tmp
 

config.dhall

@@ -1,95 +0,0 @@
--- convenience funcs for validation.
-let Prelude =
-      https://prelude.dhall-lang.org/v20.2.0/package.dhall
-        sha256:a6036bc38d883450598d1de7c98ead113196fe2db02e9733855668b18096f07b
-
-let NuConfig =
-    -- | define a configuration schema.
-      { Type =
-          { Port : Natural
-          , AppName : Text
-          , LiveMode : Bool
-          , DevelMode : Bool
-          , LoginCookieName : Optional Text
-          }
-      , default =
-        { Port = 3000
-        , AppName = "pcmt"
-        , LiveMode = False
-        , DevelMode = False
-        , LoginCookieName = None Text
-        }
-      }
-
-let NuConfig/validate
-    : NuConfig.Type -> Type
-    =
-      -- | define validation.
-      \(c : NuConfig.Type) ->
-        let expected = { validPort = True }
-
-        let actual =
-              { validPort =
-                  -- | make sure port number belongs to the <1;65535> range.
-                      Prelude.Natural.lessThanEqual 1 c.Port
-                  &&  Prelude.Natural.lessThanEqual c.Port 65535
-              }
-
-        in  expected === actual
-
-let nuconf = NuConfig::{ LiveMode = True, DevelMode = True }
-
-let _ =
-    -- | validate the configuration.
-      assert : NuConfig/validate nuconf
-
-let Config =
-    -- | define configuration schema.
-      { Host : Text
-      , Port : Natural
-      , AppName : Text
-      , LiveMode : Bool
-      , DevelMode : Bool
-      , SessionCookieName : Text
-      , SessionCookieSecret : Text
-      }
-
-let defconf
-    -- | a full default configuration.
-    -- | TODO: have this reside on the Internet and import it similar to how
-    -- | the Dhall Prelude is imported.
-    : Config
-    = { Host = ""
-      , Port = 3000
-      , AppName = "pcmt"
-      , LiveMode = False
-      , DevelMode = False
-      , SessionCookieName = "sessionz"
-      , SessionCookieSecret = "secret"
-      }
-
-let conf
-    : Config
-    = defconf // { LiveMode = True, DevelMode = False }
-
-let Config/validate
-    : Config -> Type
-    =
-      -- | define validation.
-      \(config : Config) ->
-        let expected = { validPort = True }
-
-        let actual =
-              { validPort =
-                  -- | make sure port number belongs to the <1;65535> range.
-                      Prelude.Natural.lessThanEqual 1 config.Port
-                  &&  Prelude.Natural.lessThanEqual config.Port 65535
-              }
-
-        in  expected === actual
-
-let _ =
-    -- | validate the configuration.
-      assert : Config/validate conf
-
-in  conf

exampleConfig.dhall

@@ -0,0 +1,22 @@
+{- import config schema that is integrity-check protected -}
+let ConfigSchema =
+      https://git.dotya.ml/mirre-mt/pcmt/raw/branch/development/config/schema/package.dhall
+        sha256:ad7ba86d5d388a99b7543faa0e4c81ba1d9b78fa6c32fdaf4ac4477089d177be
+
+let Config = ConfigSchema.Schema
+
+let c =
+    {- below we allow overriding config values using env vars -}
+      Config::{
+      , Host = env:PCMT_HOST as Text ? Config.default.Host
+      , LiveMode = env:PCMT_LIVE ? True
+      , DevelMode = env:PCMT_DEVEL ? True
+      }
+
+let _ =
+    {- validate that the above adheres to the config schema and contains valid
+       entries (not everything gets validated though).
+    -}
+      assert : ConfigSchema.Schema/validate c
+
+in  c