refactor(users/Profpatsch/aerc): move ini stuff out

First shot at generating a dhall FFI standard.

Change-Id: I1cdf7eeaa6b2668a49282315f308a8e51abd0cf6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5887
Reviewed-by: Profpatsch <mail@profpatsch.de>
Tested-by: BuildkiteCI

users/Profpatsch/ini/default.nix

@@ -0,0 +1,6 @@
+{ depot, ... }:
+{
+  externs = {
+    renderIni = depot.users.Profpatsch.toINI { };
+  };
+}

users/Profpatsch/ini/ini.dhall

@@ -0,0 +1,36 @@
+let lib = ../dhall/lib.dhall
+
+let NameVal = λ(T : Type) → { name : Text, value : T }
+
+let ValueList = λ(T : Type) → List (NameVal T)
+
+let Section = ValueList Text
+
+let Sections = ValueList Section
+
+let Ini = { globalSection : Section, sections : Sections }
+
+let
+    -- Takes to INI files and merges their global sections and their section lists,
+    -- without duplicating by section name.
+    appendInis =
+      λ(inis : List Ini) →
+          { globalSection =
+              lib.List/concat
+                (NameVal Text)
+                (lib.List/map Ini Section (λ(i : Ini) → i.globalSection) inis)
+          , sections =
+              lib.List/concat
+                (NameVal Section)
+                (lib.List/map Ini Sections (λ(i : Ini) → i.sections) inis)
+          }
+        : Ini
+
+let
+    -- Signatures of functions that are input via FFI.
+    Externs =
+      { -- given a dsl of functions to create an Ini, render the ini file
+        renderIni : Ini → Text
+      }
+
+in  { NameVal, ValueList, Section, Sections, Ini, appendInis, Externs }