commit d65261475579ff8e1be68d45c6f209767ea29df3
Author: Jamie Willis <j.willis19@imperial.ac.uk>
Date:   Fri Aug 20 15:24:06 2021 +0100

    Initial repo, can someone write the parser?

diff --git a/.gitignore b/.gitignore
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+++ b/.gitignore
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+*.hi
+*.o
+*.svg
+/dist-newstyle/
+*.dump*
+*/dump-core/
+.vscode/
+*.hp
+*.prof
+*.verbose-core2core
+log
+.ghc.environment*
+hie.*
+*.c
+*.h
+cabal.project.local
\ No newline at end of file
diff --git a/CHANGELOG.md b/CHANGELOG.md
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+++ b/CHANGELOG.md
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+# Revision history for design-patterns-for-parser-combinators
+
+## 0.1.0.0 -- YYYY-mm-dd
+
+* First version. Released on an unsuspecting world.
diff --git a/LICENSE b/LICENSE
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--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,30 @@
+Copyright (c) 2021, Jamie Willis
+
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+    * Redistributions of source code must retain the above copyright
+      notice, this list of conditions and the following disclaimer.
+
+    * Redistributions in binary form must reproduce the above
+      copyright notice, this list of conditions and the following
+      disclaimer in the documentation and/or other materials provided
+      with the distribution.
+
+    * Neither the name of Jamie Willis nor the names of other
+      contributors may be used to endorse or promote products derived
+      from this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
\ No newline at end of file
diff --git a/cabal.project b/cabal.project
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index 0000000..8bdd5cd
--- /dev/null
+++ b/cabal.project
@@ -0,0 +1,3 @@
+packages: .
+optimization:2
+profiling-detail:none
\ No newline at end of file
diff --git a/design-patterns-for-parser-combinators.cabal b/design-patterns-for-parser-combinators.cabal
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--- /dev/null
+++ b/design-patterns-for-parser-combinators.cabal
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+cabal-version:      3.4
+name:               design-patterns-for-parser-combinators
+version:            0.1.0.0
+
+-- A short (one-line) description of the package.
+-- synopsis:
+
+-- A longer description of the package.
+-- description:
+
+license:            BSD-3-Clause
+license-file:       LICENSE
+author:             Jamie Willis and Nicolas Wu
+maintainer:         j.willis19@imperial.ac.uk
+
+extra-source-files: CHANGELOG.md
+
+executable design-patterns-for-parser-combinators
+    main-is:          Main.hs
+
+    -- Modules included in this executable, other than Main.
+    other-modules:    Miniparsec,
+                      Miniparsec.Impl,
+                      AST,
+                      Interpreter,
+                      Parser
+
+    build-depends:    base ^>=4.14.2.0,
+                      selective,
+                      containers
+    hs-source-dirs:   src
+    default-language: Haskell2010
diff --git a/src/AST.hs b/src/AST.hs
new file mode 100644
index 0000000..8c73f60
--- /dev/null
+++ b/src/AST.hs
@@ -0,0 +1,9 @@
+module AST where
+
+data Expr = Add Expr Expr
+          | Sub Expr Expr
+          | Mul Expr Expr
+          | Neg Expr
+          | Num Int
+          | Var String
+
diff --git a/src/Interpreter.hs b/src/Interpreter.hs
new file mode 100644
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--- /dev/null
+++ b/src/Interpreter.hs
@@ -0,0 +1,15 @@
+module Interpreter where
+
+import AST
+
+{-|
+The backbone of our fancy interpreter, evaluates an expression with a context
+to produce a final value.
+-}
+eval :: Expr -> (String -> Int) -> Int
+eval (Add x y) = (+) <$> eval x <*> eval y
+eval (Sub x y) = (-) <$> eval x <*> eval y
+eval (Mul x y) = (*) <$> eval x <*> eval y
+eval (Neg x)   = negate <$> eval x
+eval (Num n)   = pure n
+eval (Var x)   = ($ x)
\ No newline at end of file
diff --git a/src/Main.hs b/src/Main.hs
new file mode 100644
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--- /dev/null
+++ b/src/Main.hs
@@ -0,0 +1,19 @@
+module Main where
+
+import Interpreter
+import Parser
+
+calc :: String -> (String -> Int) -> Int
+calc e = eval (parseExpr e)
+
+main :: IO ()
+main = do
+  putStrLn "This is the Really Amazing Calculator"
+  putStrLn "Enter an equation, variables x1, xy, and z are available"
+  eq <- getLine
+  print $ calc eq ctx
+  where
+    ctx :: String -> Int
+    ctx "x1" = 7
+    ctx "xy" = 2
+    ctx "z"  = 42
diff --git a/src/Miniparsec.lhs b/src/Miniparsec.lhs
new file mode 100644
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--- /dev/null
+++ b/src/Miniparsec.lhs
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+> module Miniparsec (
+>    module Miniparsec,
+>    module Miniparsec.Impl,
+>    module Data.Functor,
+>    module Control.Applicative,
+>    module Control.Selective
+>  ) where
+
+> import Miniparsec.Impl
+> import Data.Functor
+> import Control.Applicative
+> import Control.Selective
+> import Data.Foldable (asum)
+
+> infixr 4 <:>
+> (<:>) :: Applicative f => f a -> f [a] -> f [a]
+> (<:>) = liftA2 (:)
+
+> infixl 4 <~>
+> (<~>) :: Applicative f => f a -> f b -> f (a, b)
+> (<~>) = liftA2 (,)
+
+> choice :: Alternative f => [f a] -> f a
+> choice = asum
+
+> infixl 3 >?>
+> (>?>) :: (Selective f, Alternative f) => f a -> (a -> Bool) -> f a
+> fx >?> p = select ((\x -> if p x then Right x else Left ()) <$> fx) empty
+
+> filteredBy :: (Selective f, Alternative f) => f a -> (a -> Bool) -> f a
+> filteredBy = (>?>)
+
+> char :: Char -> Parser Char
+> char c = satisfy (== c) <?> show [c]
+
+> string :: String -> Parser String
+> string str = traverse char str <?> show str
+
+> item :: Parser Char
+> item = satisfy (const True) <?> "any character"
+
+> oneOf :: [Char] -> Parser Char
+> oneOf = choice . map char
+
+> noneOf :: [Char] -> Parser Char
+> noneOf = satisfy . (not .) . flip elem
+
+> eof :: Parser ()
+> eof = notFollowedBy item <?> "end of file"
+
+> pos :: Parser (Int, Int)
+> pos = line <~> col
\ No newline at end of file
diff --git a/src/Miniparsec/Impl.lhs b/src/Miniparsec/Impl.lhs
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--- /dev/null
+++ b/src/Miniparsec/Impl.lhs
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+> {-# OPTIONS_GHC -Wno-incomplete-patterns #-}
+> {-# LANGUAGE RankNTypes #-}
+> {-# LANGUAGE GeneralizedNewtypeDeriving #-}
+> {-# LANGUAGE RecordWildCards #-}
+> {-# LANGUAGE InstanceSigs #-}
+> {-# LANGUAGE NamedFieldPuns #-}
+> {-# LANGUAGE FlexibleInstances #-}
+> module Miniparsec.Impl (
+>   Parser, parse,
+>   satisfy, try,
+>   lookAhead, notFollowedBy,
+>   unexpected, fail, (<?>),
+>   line, col
+> ) where
+
+> import Prelude hiding (fail)
+> import Data.Set (Set)
+> import qualified Data.Set as Set
+> import Data.List (intercalate, intersperse, elemIndices)
+> import Data.Maybe (catMaybes)
+> import Data.Function (on)
+> import Control.Applicative (Applicative(liftA2), Alternative(empty, (<|>)))
+> import Control.Selective ( Selective(..))
+
+This implementation of a parser combinator library is based on the error semantics of
+   https://github.com/j-mie6/Parsley, which is a refined version of
+   https://hackage.haskell.org/package/megaparsec errors. The code for these projects is freely
+   available for use here under the following licenses:
+Copyright © 2015–present Megaparsec contributors
+Copyright © 2007 Paolo Martini
+Copyright © 1999–2000 Daan Leijen
+All rights reserved.
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS “AS IS” AND ANY EXPRESS
+OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN
+NO EVENT SHALL THE COPYRIGHT HOLDERS BE LIABLE FOR ANY DIRECT, INDIRECT,
+INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA,
+OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE,
+EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+Copyright (c) 2020, Jamie Willis
+All rights reserved.
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+1. Redistributions of source code must retain the above copyright notice, this
+   list of conditions and the following disclaimer.
+2. Redistributions in binary form must reproduce the above copyright notice,
+   this list of conditions and the following disclaimer in the documentation
+   and/or other materials provided with the distribution.
+3. Neither the name of the copyright holder nor the names of its
+   contributors may be used to endorse or promote products derived from
+   this software without specific prior written permission.
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+
+> data Pos = Pos {
+>   _line :: Int,
+>   _col :: Int
+> }
+
+> instance Eq Pos where
+>   (==) :: Pos -> Pos -> Bool
+>   err1 == err2 = _line err1 == _line err2 && _col err1 == _col err2
+
+> instance Ord Pos where
+>   (<=) :: Pos -> Pos -> Bool
+>   err1 <= err2 = _line err1 < _line err2 || (_line err1 == _line err2 && _col err1 <= _col err2)
+
+> instance Show Pos where
+>   show Pos{..} = "(" ++ show _line ++ ", " ++ show _col ++ ")"
+
+> data Input = Input {
+>   str :: String,
+>   _pos :: Pos
+> }
+
+> data Error = Oops {
+>   expected :: Set Item,
+>   unexpect :: Maybe Item,
+>   msgs :: [String],
+>   errPos :: Pos
+> } deriving Show
+
+> instance Eq Error where
+>   (==) :: Error -> Error -> Bool
+>   (==) = (==) `on` errPos
+
+> instance Ord Error where
+>   (<=) :: Error -> Error -> Bool
+>   (<=) = (<=) `on` errPos
+
+> newtype Max a = Max { getMax :: a }
+> instance (Alternative f, Ord a) => Semigroup (Max (f a)) where
+>   Max m <> Max n = Max $ (max <$> m <*> n) <|> m <|> n
+
+> instance Semigroup Error where
+>   err1 <> err2
+>     | err1 == err2 = err1 {
+>         expected = expected err1 <> expected err2,
+>         unexpect = getMax (Max (unexpect err1) <> Max (unexpect err2)),
+>         msgs = msgs err1 ++ msgs err2
+>       }
+>     | err1 > err2 = err1
+>     | err1 < err2 = err2
+
+> -- Based on the Parsley's error format
+> format :: String -> Error -> String
+> format input Oops{..} =
+>   let preamble         = Just $ show errPos ++ ":"
+>       unexpectLine     = ("unexpected " ++) . show <$> unexpect
+>       expectedLine     = fmap ("expected " ++) $ foldMap Just $ intersperse ", " $ map show $ Set.toList expected
+>       inputLines       = lines input
+>       problem          = Just $ "> " ++ if null inputLines then "" else inputLines !! (_line errPos - 1)
+>       caret            = Just $ "  " ++ replicate (_col errPos - 1) ' ' ++ "^"
+>       joinTogether p q = liftA2 (\x y -> x ++ " " ++ y) p q <|> p <|> q
+>   in  intercalate "\n  " (catMaybes ([preamble `joinTogether` unexpectLine, expectedLine] ++ map Just msgs ++ [problem, caret]))
+
+> data Item = Raw String | Named String | EndOfInput deriving (Eq, Ord)
+
+> instance Show Item where
+>   show EndOfInput = "end of input"
+>   show (Raw " ") = "space"
+>   show (Raw "\n") = "newline"
+>   show (Raw "\t") = "tab"
+>   show (Raw raw) = show (takeWhile (/= ' ') raw)
+>   show (Named named) = named
+
+> newtype Hints = Hints [Set Item] deriving (Semigroup, Monoid)
+
+> data State a r = State {
+>   input :: Input,
+>   good :: a -> Input -> Hints -> Either String r,
+>   bad :: Error -> Input -> Either String r
+> }
+> newtype Parser a = Parser (forall r. State a r -> Either String r)
+
+> parse :: Parser a -> String -> Either String a
+> parse (Parser k) input = k $ State {
+>     input = Input {str = input, _pos = Pos {_line = 1, _col = 1}},
+>     good = \x _ _ -> Right x,
+>     bad = \err _ -> Left (format input err)
+>   }
+
+> instance Functor Parser where
+>   fmap :: (a -> b) -> Parser a -> Parser b
+>   fmap f (Parser k) =
+>     Parser $ \st@State{good} -> k (st {
+>       good = good . f
+>     })
+
+> instance Applicative Parser where
+>   pure :: a -> Parser a
+>   pure x = Parser $ \st@State{good, input} -> good x input mempty
+>
+>   liftA2 :: (a -> b -> c) -> Parser a -> Parser b -> Parser c
+>   liftA2 f (Parser p) (Parser q) = Parser $ \st@State{..} ->
+>     let doQ x input hs = q (State {
+>         good = \y input' hs' -> good (f x y) input' (combineHints hs hs' (_pos input) (_pos input')),
+>         bad = \err input' -> bad (withHints err hs (_pos input) (_pos input')) input',
+>         input = input
+>       })
+>     in p (st {good = doQ})
+
+> instance Selective Parser where
+>   select :: Parser (Either a b) -> Parser (a -> b) -> Parser b
+>   select (Parser p) (Parser q) = Parser $ \st@State{..} ->
+>     let handle (Left x) = \input hs -> q (State {
+>           good = \f input' hs' -> good (f x) input' (combineHints hs hs' (_pos input) (_pos input')),
+>           bad = \err input' -> bad (withHints err hs (_pos input) (_pos input')) input',
+>           input = input
+>         })
+>         handle (Right x) = good x
+>     in p (st {good = handle})
+
+> instance Alternative Parser where
+>   empty :: Parser a
+>   empty = Parser $ \st@State{bad, input} -> bad (Oops {
+>       expected = Set.empty,
+>       unexpect = Nothing,
+>       msgs = [],
+>       errPos = _pos input
+>     }) input
+
+>   (<|>) :: Parser a -> Parser a -> Parser a
+>   Parser p <|> Parser q = Parser $ \st@State{..} ->
+>     let doQ err input'
+>           | _pos input < _pos input'  = bad err input'
+>           | _pos input == _pos input' = q (st {
+>               good = \x input' hs ->
+>                 if _pos input == _pos input' then good x input' (toHints err (_pos input') <> hs)
+>                 else                         good x input' hs,
+>               bad = \err' -> bad (err <> err')
+>             })
+>     in p (st {bad = doQ})
+
+> satisfy :: (Char -> Bool) -> Parser Char
+> satisfy f = Parser $ \st@State{..} -> case str input of
+>   c:cs | f c -> let p@Pos{..} = _pos input in case c of
+>     '\n' -> good '\n' (input {str = cs, _pos = p {_col = 1, _line = _line + 1}}) mempty
+>     c    -> good c (input {str = cs, _pos = p {_col = _col + 1}}) mempty
+>   cs         -> bad (Oops {
+>     expected = Set.empty,
+>     unexpect = Just (foldr (const . Raw . pure) EndOfInput cs),
+>     msgs = [],
+>     errPos = _pos input
+>   }) input
+
+> try :: Parser a -> Parser a
+> try (Parser p) = Parser $ \st@State{..} -> p (st {bad = \err _ -> bad err input})
+
+> lookAhead :: Parser a -> Parser a
+> lookAhead (Parser p) = Parser $ \st@State{..} -> p (st {good = \x _ _ -> good x input mempty})
+
+> notFollowedBy :: Parser a -> Parser ()
+> notFollowedBy (Parser p) = Parser $ \st@State{..} ->
+>   let oldPos = _pos input
+>       item newPos = take (_col newPos - _col oldPos)
+>                   $ head (lines (str input))
+>       err input' = Oops {
+>           expected = Set.empty,
+>           unexpect = Just $ if null (str input) then EndOfInput else Raw (item (_pos input')),
+>           msgs = [],
+>           errPos = oldPos
+>         }
+>   in p (st {
+>     good = \_ input' _ -> bad (err input') input,
+>     bad = \_ _ -> good () input mempty
+>   })
+
+> unexpected :: String -> Parser a
+> unexpected msg = Parser $ \st@State{bad, input} -> bad (Oops {
+>     expected = Set.empty,
+>     unexpect = Just (Named msg),
+>     msgs = [],
+>     errPos = _pos input
+>   }) input
+
+> fail :: String -> Parser a
+> fail msg = Parser $ \st@State{bad, input} -> bad (Oops {
+>     expected = Set.empty,
+>     unexpect = Nothing,
+>     msgs = [msg],
+>     errPos = _pos input
+>   }) input
+
+> line :: Parser Int
+> line = Parser $ \State{good, input} -> good (_line (_pos input)) input mempty
+
+> col :: Parser Int
+> col = Parser $ \State{good, input} -> good (_col (_pos input)) input mempty
+
+> infix 0 <?>
+> (<?>) :: Parser a -> String -> Parser a
+> Parser p <?> label = Parser $ \st@State{..} ->
+>   let label'
+>         | null label = Nothing
+>         | otherwise  = Just (Named label)
+>       hintFix x input' hs
+>         | _pos input < _pos input', Nothing <- label' = good x input' (refreshLastHint hs Nothing)
+>         | _pos input < _pos input'                    = good x input' hs
+>         | _pos input == _pos input'                   = good x input' (refreshLastHint hs label')
+>       labelApply err input' = flip bad input' $
+>         if _pos input == _pos input' then err { expected = maybe Set.empty Set.singleton label' }
+>         else                              err
+>   in p (st {good = hintFix, bad = labelApply})
+
+> combineHints :: Hints -> Hints -> Pos -> Pos -> Hints
+> combineHints hs hs' pos pos'
+>   | pos == pos' = hs <> hs'
+>   | pos < pos'  = hs'
+>   | otherwise   = error (show pos ++ " is not <= " ++ show pos')
+
+> withHints :: Error -> Hints -> Pos -> Pos -> Error
+> withHints err (Hints hs) pos pos'
+>   | pos == pos' = err { expected = Set.unions (expected err : hs) }
+>   | pos < pos'  = err
+>   | otherwise   = error (show pos ++ " is not <= " ++ show pos')
+
+> -- Taken from megaparsec
+> refreshLastHint :: Hints -> Maybe Item -> Hints
+> refreshLastHint (Hints [])       _        = Hints []
+> refreshLastHint (Hints (_ : hs)) Nothing  = Hints hs
+> refreshLastHint (Hints (_ : hs)) (Just h) = Hints (Set.singleton h : hs)
+
+> -- Taken from megaparsec
+> toHints :: Error -> Pos -> Hints
+> toHints err@Oops{..} pos
+>   | errPos == pos = Hints [expected | not (Set.null expected)]
+>   | otherwise     = mempty
\ No newline at end of file
diff --git a/src/Parser.hs b/src/Parser.hs
new file mode 100644
index 0000000..ed049be
--- /dev/null
+++ b/src/Parser.hs
@@ -0,0 +1,11 @@
+module Parser where
+
+import AST
+import Miniparsec
+
+{-|
+This function should turn a string into an expression.
+-}
+-- Should be a good issue for a newcomer?
+parseExpr :: String -> Expr
+parseExpr = error "Not implemented yet :("
\ No newline at end of file