docs: document extra_args provider field

Problem: Overriding a single flag (e.g. `-outdir=build`) required
redefining the entire `args` function, duplicating all preset defaults.

Solution: Add `extra_args` field that appends to the resolved `args`
after evaluation. Accepts a static table or a context function.

README.md

@@ -1,6 +1,6 @@
 # preview.nvim
 
-**Universal document previewer for Neovim**
+**Universal previewer for Neovim**
 
 An extensible framework for compiling and previewing _any_ documents (LaTeX,
 Typst, Markdown, etc.)—diagnostics included.

doc/preview.txt

@@ -68,6 +68,10 @@ Provider fields: ~
                                         receives a |preview.Context| and
                                         returns a string[].
 
+  {extra_args}       (string[]|function) Appended to {args} after evaluation.
+                                        Useful for adding flags to a preset
+                                        without replacing its defaults.
+
   {cwd}              (string|function)  Working directory. If a function,
                                         receives a |preview.Context|.
                                         Default: git root or file directory.

flake.nix

@@ -19,9 +19,10 @@
     {
       formatter = forEachSystem (pkgs: pkgs.nixfmt-tree);
 
-      devShells = forEachSystem (pkgs: {
+      devShells = forEachSystem (
-        default = pkgs.mkShell {
+        pkgs:
-          packages = [
+        let
+          devTools = [
             (pkgs.luajit.withPackages (
               ps: with ps; [
                 busted
@@ -32,10 +33,25 @@
             pkgs.stylua
             pkgs.selene
             pkgs.lua-language-server
+          ];
+        in
+        {
+          default = pkgs.mkShell {
+            packages = devTools;
+          };
+          presets = pkgs.mkShell {
+            packages = devTools ++ [
+              pkgs.typst
+              pkgs.texliveMedium
+              pkgs.tectonic
+              pkgs.pandoc
+              pkgs.asciidoctor
+              pkgs.quarto
               pkgs.plantuml
               pkgs.mermaid-cli
             ];
           };
-      });
+        }
+      );
     };
 }

lua/preview/compiler.lua

@@ -309,6 +309,9 @@ function M.compile(bufnr, name, provider, ctx, opts)
   if provider.args then
     vim.list_extend(cmd, eval_list(provider.args, resolved_ctx))
   end
+  if provider.extra_args then
+    vim.list_extend(cmd, eval_list(provider.extra_args, resolved_ctx))
+  end
 
   log.dbg('compiling buffer %d with provider "%s": %s', bufnr, name, table.concat(cmd, ' '))
 

lua/preview/init.lua

@@ -2,6 +2,7 @@
 ---@field ft? string
 ---@field cmd string[]
 ---@field args? string[]|fun(ctx: preview.Context): string[]
+---@field extra_args? string[]|fun(ctx: preview.Context): string[]
 ---@field cwd? string|fun(ctx: preview.Context): string
 ---@field env? table<string, string>
 ---@field output? string|fun(ctx: preview.Context): string
@@ -94,6 +95,7 @@ function M.setup(opts)
     vim.validate(prefix .. '.cmd', provider.cmd, 'table')
     vim.validate(prefix .. '.cmd[1]', provider.cmd[1], 'string')
     vim.validate(prefix .. '.args', provider.args, { 'table', 'function' }, true)
+    vim.validate(prefix .. '.extra_args', provider.extra_args, { 'table', 'function' }, true)
     vim.validate(prefix .. '.cwd', provider.cwd, { 'string', 'function' }, true)
     vim.validate(prefix .. '.output', provider.output, { 'string', 'function' }, true)
     vim.validate(prefix .. '.error_parser', provider.error_parser, 'function', true)

lua/preview/presets.lua

@@ -115,6 +115,24 @@ local function parse_asciidoctor(output)
   return diagnostics
 end
 
+---@param output string
+---@return preview.Diagnostic[]
+local function parse_mermaid(output)
+  local lnum = output:match('Parse error on line (%d+)')
+  if not lnum then
+    return {}
+  end
+  local msg = output:match('(Expecting .+)') or 'parse error'
+  return {
+    {
+      lnum = tonumber(lnum) - 1,
+      col = 0,
+      message = msg,
+      severity = vim.diagnostic.severity.ERROR,
+    },
+  }
+end
+
 ---@type preview.ProviderConfig
 M.typst = {
   ft = 'typst',
@@ -313,19 +331,7 @@ M.mermaid = {
     return (ctx.file:gsub('%.mmd$', '.svg'))
   end,
   error_parser = function(output)
-    local diagnostics = {}
+    return parse_mermaid(output)
-    for line in output:gmatch('[^\r\n]+') do
-      local lnum = line:match('^%s*Parse error on line (%d+)')
-      if lnum then
-        table.insert(diagnostics, {
-          lnum = tonumber(lnum) - 1,
-          col = 0,
-          message = line,
-          severity = vim.diagnostic.severity.ERROR,
-        })
-      end
-    end
-    return diagnostics
   end,
   clean = function(ctx)
     return { 'rm', '-f', (ctx.file:gsub('%.mmd$', '.svg')) }