A quick mock-up of a spreadsheet program's structure - because why not

Spreadsheet.hs

{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE GADTs #-}
module Spreadsheet where

import Data.Text
import Data.Word
import Data.Vector

newtype Row = Row Word32
newtype Col = Col Word32

--------------------------------------------------------------------------------
--                       Spreadsheet Language                                 --
--------------------------------------------------------------------------------

-- catch some errors at compile-time with GADTs
-- e.g., never mix up bool and numeric ops during eval
-- no text support yet
data Expr a where
  -- numeric operators
  Add              :: Expr Double -> Expr Double -> Expr Double -- expr + expr
  Subtract         :: Expr Double -> Expr Double -> Expr Double -- expr - expr
  Multiply         :: Expr Double -> Expr Double -> Expr Double -- expr * expr
  Divide           :: Expr Double -> Expr Double -> Expr Double -- expr / expr
  Power            :: Expr Double -> Expr Double -> Expr Double -- expr ^ expr
  Sin              :: Expr Double -> Expr Double                -- sin(expr)
  Negate           :: Expr Double -> Expr Double                -- -expr
  Cos              :: Expr Double -> Expr Double                -- cos(expr)
  Tan              :: Expr Double -> Expr Double                -- tan(expr)

  -- array operators
  Sum              :: [Expr Double] -> Expr Double              -- sum (expr, expr, expr, ...)
  Any              :: [Expr Bool] -> Expr Bool                  -- any (expr, expr, expr, ...)
  All              :: [Expr Bool] -> Expr Bool                  -- all (expr, expr, expr, ...)
  Product          :: [Expr Double] -> Expr Double              -- product (expr, expr, expr, ...)

  -- boolean operators
  LessThan         :: Expr Double-> Expr Double -> Expr Bool    -- expr < expr
  GreaterThan      :: Expr Double-> Expr Double -> Expr Bool    -- expr > expr
  EqualTo          :: Expr Double-> Expr Double -> Expr Bool    -- expr == expr
  LessThanEqual    :: Expr Double-> Expr Double -> Expr Bool    -- expr <= expr
  GreaterThanEqual :: Expr Double-> Expr Double -> Expr Bool    -- expr >= expr
  And              :: Expr Bool-> Expr Bool -> Expr Bool        -- expr & expr
  Or               :: Expr Bool -> Expr Bool -> Expr Bool       -- expr | expr

  -- operational ops: affect env
  Assign           :: Expr a -> Expr a                          -- = <expr>
  Parens           :: Expr a -> Expr a                          -- (expr)

  -- Terminal nodes
  Number           :: Double -> Expr Double                     -- a number
  Reference        :: Row -> Col -> Expr a                      -- $C1
  Boolean          :: Bool -> Expr Bool                         -- true | false

--------------------------------------------------------------------------------
--                        Spreadsheet State                                   --
--------------------------------------------------------------------------------
data Spreadsheet
  = Spreadsheet (Vector Cell)  -- 2D structure flattened to 1D


data Cell
  = Empty
  | forall a. Filled (Expr a)  -- lazily evaluated cells


--------------------------------------------------------------------------------
--                        Application Model                                   --
--                          (Assumes GUI)                                     --
--------------------------------------------------------------------------------
type TextStart = Word32
type TextEnd = Word32
type CursorPos = Word32
type StartRow = Row
type EndRow = Row
type StartCol = Col
type EndCol = Col
type TopLeft = (Row,Col)
type Width = Word32
type Height = Word32
type RegionSize = (Width,Height)
data Region = Region TopLeft RegionSize


data Action

  -- valid actions while in a text input box for a cell; edit mode
  = Input Char
  | DeleteAtPoint Text CursorPos
  | BackspaceAtPoint Text CursorPos
  | DeleteWordForward Text CursorPos
  | DeleteWordBackwards Text CursorPos
  | SelectText TextStart TextEnd
  | SelectAll
  | CopyText TextStart TextEnd
  | CutText TextStart TextEnd
  | Paste Text CursorPos
  | FinalizeEdit

  -- valid actions while on a cell; sheet mode
  -- leaving off paste functionality for now
  | StartEdit Row Col
  | ClearCell Row Col
  | CopyCell Row Col
  | SelectColumn Col
  | SelectRow Row
  | SelectRegion Region
  | ClearCells Region
  | CopyCells Region

  -- movement over cells; sheet mode
  | MoveRight
  | MoveLeft
  | MoveUp
  | MoveDown

  -- auxiliary operations; sheet mode
  | Quit
  | LoadFrom FilePath
  | SaveCurrent
  | SaveTo FilePath

--------------------------------------------------------------------------------
--                              Parser                                        --
--------------------------------------------------------------------------------
data ParseError

parseExpr :: String -> Either ParseError (Expr a)
parseExpr = undefined