ghc: Init

This only adds the configuration, not the packages, because they are
massive.

users/simon/modules/default.nix

@@ -3,6 +3,7 @@
     ./alacritty.nix
     ./anki
     ./games.nix
+    ./ghci.nix
     ./git.nix
     ./gpg.nix
     ./gtk.nix

users/simon/modules/ghci.nix

@@ -0,0 +1,32 @@
+{ pkgs, ... }:
+
+{
+  home.file.".ghc/ghci.conf".text = /* haskell */ ''
+    :seti -XOverloadedStrings
+
+    -- Lambdabot (from https://wiki.haskell.org/GHC/GHCi#Lambdabot_Commands)
+    dotGHCI_escapeShellArg arg = "'" ++ concatMap (\c -> if c == '\''' then "'\\'''" else [c]) arg ++ "'"
+    lb s1 s2 = return $ ":!lambdabot -n -e " ++ dotGHCI_escapeShellArg s1 ++ "\\ " ++ dotGHCI_escapeShellArg s2
+    :def! lb lb ""                   -- runs arbitrary lambdabot commands
+    :def! pl lb "pl"                 -- converts code to point-free (aka pointless) form
+    :def! unpl lb "unpl"             -- converts back from point-free (aka pointless) form
+    :def! do lb "do"                 -- converts binds to do notation
+    :def! undo lb "undo"             -- converts do blocks to bind notation
+    :def! index lb "index"           -- finds the module that defines the given identifier
+    :def! instances lb "instances"   -- finds all instances of a given type class
+    :def! src lb "src"               -- tries to find the source code for the given identifier
+    :def! oeis lb "oeis"             -- looks up the On-Line Encyclopedia of Integer Sequences (https://oeis.org/)
+  '';
+
+  home.file.".haskeline".text = ''
+    completionType: MenuCompletion
+    maxHistorySize: Just 10000
+    historyDuplicates: IgnoreAll
+  '';
+
+  home.packages = with pkgs; [
+    # GHC has a huge closure size, instead use nix-shell (ad-hoc or per-project)
+    #ghc
+    #lambdabot
+  ];
+}