Add a cmd quasiquoter

src/Streamly/Internal/System.hs

@@ -0,0 +1,138 @@
+-- |
+-- Module      : Streamly.Internal.System
+-- Copyright   : (c) 2022 Composewell Technologies
+-- License     : Apache-2.0
+-- Maintainer  : [email protected]
+-- Stability   : experimental
+-- Portability : GHC
+--
+{-# LANGUAGE TemplateHaskell #-}
+
+module Streamly.Internal.System
+( cmd
+, trim
+)
+where
+
+import Control.Applicative (Alternative(..))
+import Control.Exception (displayException)
+import Data.Functor.Identity (runIdentity)
+import Streamly.Internal.Data.Parser (Parser)
+
+import Language.Haskell.TH
+import Language.Haskell.TH.Quote
+
+import qualified Streamly.Data.Fold as Fold
+import qualified Streamly.Internal.Data.Parser as Parser
+    (some, many, takeWhile1)
+import qualified Streamly.Data.Stream as Stream  (fromList, parse)
+import qualified Streamly.Internal.Unicode.Parser as Parser
+
+data StrSegment
+    = StrText String
+    | StrVar String
+    deriving (Show, Eq)
+
+formatSpace :: String -> String
+formatSpace = foldr go ""
+  where
+    go x acc = x:if x == ' ' then dropWhile (' ' ==) acc else acc
+
+-- | Replace a newline by a space and convert multiple spaces to single space
+--
+-- >>> :set -XQuasiQuotes
+-- >>> import Streamly.Internal.System
+-- >>> trim "  abc   \n   bbb  \n  ccc  "
+-- " abc bbb ccc "
+--
+trim :: String -> String
+trim = formatSpace <$> (unwords . fmap formatSpace . lines)
+
+haskellIdentifier :: Monad m => Parser Char m String
+haskellIdentifier =
+    let p = Parser.alphaNum <|> Parser.char '\'' <|> Parser.char '_'
+     in Parser.some p Fold.toList
+
+strParser :: Monad m => Parser Char m [StrSegment]
+strParser = Parser.many content Fold.toList
+
+    where
+
+    plainText = StrText . trim <$> Parser.takeWhile1 (/= '#') Fold.toList
+    escHash = StrText . (: []) <$> (Parser.char '#' *> Parser.char '#')
+    lineCont = StrText [] <$ (Parser.char '#' *> Parser.char '\n')
+    var = StrVar <$>
+            (  Parser.char '#'
+            *> Parser.char '{'
+            *> haskellIdentifier
+            <* Parser.char '}'
+            )
+    plainHash = StrText . (: []) <$> Parser.char '#'
+
+    -- order is important
+    content = plainText <|> escHash <|> lineCont <|> var <|> plainHash
+
+strSegmentExp :: StrSegment -> Q Exp
+strSegmentExp (StrText text) = stringE text
+strSegmentExp (StrVar name) = do
+    valueName <- lookupValueName name
+    case valueName of
+        Just vn -> varE vn
+        Nothing ->
+            fail
+                $ "cmd quote: Haskell symbol `" ++ name
+                ++ "` is not in scope"
+
+strExp :: [StrSegment] -> Q Exp
+strExp xs = appE [| concat |] $ listE $ map strSegmentExp xs
+
+expandVars :: String -> Q Exp
+expandVars ln =
+    case runIdentity $ Stream.parse strParser (Stream.fromList ln) of
+        Left e ->
+            fail $ "cmd QuasiQuoter parse error: " ++ displayException e
+        Right x ->
+            strExp x
+
+-- | A QuasiQuoter that treats the input as a string literal:
+--
+-- >>> [cmd|x|]
+-- "x"
+--
+-- Any @#{symbol}@ is replaced by the value of the Haskell symbol @symbol@
+-- which is in scope:
+--
+-- >>> x = "hello"
+-- >>> [cmd|#{x} world!|]
+-- "hello world!"
+--
+-- @##@ means a literal @#@ without the special meaning for referencing
+-- haskell symbols:
+--
+-- >>> [cmd|##{x} world!|]
+-- "#{x} world!"
+--
+-- A @#@ at the end of line means the line continues to the next line without
+-- introducing a newline character:
+--
+-- >>> :{
+-- [cmd|hello#
+-- world!|]
+-- :}
+-- "hello world!"
+--
+-- Bugs: because of a bug in parsers, a lone # at the end of input gets
+-- removed.
+--
+cmd :: QuasiQuoter
+cmd =
+    QuasiQuoter
+        { quoteExp = expandVars
+        , quotePat = notSupported
+        , quoteType = notSupported
+        , quoteDec = notSupported
+        }
+
+    where
+
+    notSupported = error "cmd: Not supported."

streamly-process.cabal

@@ -83,8 +83,10 @@ library
   import: compile-options, optimization-options
   hs-source-dirs: src
   exposed-modules:
+
     Streamly.System.Process
     Streamly.System.Command
+    Streamly.Internal.System
     Streamly.Internal.System.Process
     Streamly.Internal.System.Command
   if flag (use-native) && !os(windows)
@@ -96,6 +98,8 @@ library
     -- Uses internal APIs
     , streamly          == 0.9.0.*
     , streamly-core     == 0.1.0
+    , template-haskell  >= 2.14  && < 2.21
+
   if !flag(use-native)
     build-depends: process >= 1.0 && < 1.7
   else