lib: doctests: file pattern arguments, more informative output

Limiting to just the file(s) you're interested can make doctest start
much quicker. With a big caveat: you can limit the starting files,
but it will also test all other local files those import.

hledger-lib/tests/doctests.hs

@@ -1,5 +1,20 @@
+{- 
+Runs hledger doctests.
+Usage examples: in hledger source dir, 
+make ghci-doctest, :main [--verbose] [--slow] [CIFILEPATHSUBSTRINGS]
+or:
+stack test hledger-lib:test:doctests [--test-arguments '[--verbose] [--slow] [CIFILEPATHSUBSTRINGS]']
+
+Arguments are case-insensitive file path substrings.
+--verbose shows files being searched for doctests and progress while running.
+--slow reloads ghci between each test (https://github.com/sol/doctest#a-note-on-performance).
+
+-}
+
 {-# LANGUAGE PackageImports #-}
 
+import Control.Monad
+import Data.Char
 import Data.List
 import System.Environment
 import "Glob" System.FilePath.Glob
@@ -7,14 +22,35 @@ import Test.DocTest
 
 main = do
   args <- getArgs
-  fs1 <- glob "Hledger/**/*.hs"
-  fs2 <- glob "Text/**/*.hs"
-  --fs3 <- glob "other/ledger-parse/**/*.hs"
-  let fs = filter (not . isInfixOf "/.") $ ["Hledger.hs"] ++ fs1 ++ fs2
+  let
+    verbose = "--verbose" `elem` args
+    slow    = "--slow" `elem` args
+    pats    = filter (not . ("-" `isPrefixOf`)) args
+
+  -- find source files
+  sourcefiles1 <- glob "Hledger/**/*.hs"
+  sourcefiles2 <- glob "Text/**/*.hs"
+  let sourcefiles = filter (not . isInfixOf "/.") $ ["Hledger.hs"] ++ sourcefiles1 ++ sourcefiles2
+  
+  -- filter by patterns (case insensitive infix substring match)
+  let 
+    fs | null pats = sourcefiles
+       | otherwise = [f | f <- sourcefiles, let f' = map toLower f, any (`isInfixOf` f') pats']
+          where pats' = map (map toLower) pats
+    fslen = length fs
+  
+  if (null fs)
+  then do
+    putStrLn $ "No file paths found matching: " ++ unwords pats
+
+  else do
+    putStrLn $ 
+      "Loading and searching for doctests in " 
+      ++ show fslen 
+      ++ if fslen > 1 then " files, plus any files they import:" else " file, plus any files it imports:"
+    when verbose $ putStrLn $ unwords fs
+
     doctest $ 
-    -- show verbose progress output
-    (if "--verbose" `elem` args then ("--verbose" :) else id) $ 
-    -- don't reload environment per test (opposite of doctest's --fast,
-    -- https://github.com/sol/doctest#a-note-on-performance)
-    (if "--slow" `elem` args then id else ("--fast" :)) $
+      (if verbose then ("--verbose" :) else id) $ 
+      (if slow then id else ("--fast" :)) $
       fs