Parser

README.md

@@ -22,3 +22,4 @@ cabal run amnesia -- <program.amn>
 
 I highly recommend watching:
 - [Haskell for Imperative Programmers](https://www.youtube.com/watch?v=Vgu82wiiZ90&list=PLe7Ei6viL6jGp1Rfu0dil1JH1SHk9bgDV&index=1)
+- [Megaparsec tutorial](https://markkarpov.com/tutorial/megaparsec.html)

amnesia.cabal

@@ -12,6 +12,8 @@ common warnings
 executable amnesia
     import:           warnings
     main-is:          Amnesia.hs
-    build-depends:    base ^>=4.18.3.0
+    build-depends:
+      base ^>=4.18.3.0,
+      megaparsec,
     hs-source-dirs:   src
     default-language: GHC2021

src/Amnesia.hs

@@ -2,22 +2,7 @@ module Main where
 
 import System.Environment (getArgs)
 import Stacks
-
-data Instr
-  = ChoA      -- select stack A
-  | ChoB      -- select stack B
-  | ChoC      -- select stack C
-  | Set Int   -- set selected stack to specific value
-  | Add       -- A = A + B
-  | Sub       -- A = A - B
-  | Mul       -- A = A * B
-  | Div       -- A = A / B
-  | Mod       -- A = A % B
-  | Nm        -- non-zero check
-  | Disp      -- print top of stack
-  | Open      -- ":" open bracket
-  | Close     -- ";" close bracket
-  deriving(Show, Eq)
+import Parser
 
 main :: IO ()
 main = do

src/Parser.hs

@@ -0,0 +1,36 @@
+module Parser
+  ( Instr(..)
+  -- , parseProgram
+  , cho -- fixme: for testing
+  ) where
+
+import Stacks (S(..))
+import Text.Megaparsec
+import Text.Megaparsec.Char
+import Data.Void
+
+data Instr
+  = Cho S  -- choose stack A/B/C
+  | Set    -- =
+  | Add    -- +
+  | Sub    -- -
+  | Mul    -- *
+  | Div    -- /
+  | Mod    -- %
+  | Disp   -- disp
+  | Nm     -- conditional check
+  | Open   -- ':'
+  | Close  -- ';'
+  deriving(Show, Eq)
+
+type Parser = Parsec Void String
+
+cho :: Parser Instr
+cho = do
+  _ <- string "cho"
+  s <- satisfy (`elem` "ABC")
+  return $ case s of
+    'A' -> Cho A
+    'B' -> Cho B
+    'C' -> Cho C
+    _  -> error "?"