G52AFP Coursework 2 - Monadic Compiler
   
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Imperative language
-------------------

> data Prog             =  Assign Name Expr
>                       |  If Expr Prog Prog
>                       |  While Expr Prog
>                       |  Seqn [Prog]
>                          deriving Show
>
> data Expr             =  Val Int | Var Name | App Op Expr Expr
>                          deriving Show
>
> type Name             =  Char
>
> data Op               =  Add | Sub | Mul | Div
>                          deriving Show


Virtual machine
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> type Stack            =  [Int]
>
> type Mem              =  [(Name,Int)]
>
> type Code             =  [Inst]
>
> data Inst             =  PUSH Int
>                       |  PUSHV Name
>                       |  POP Name
>                       |  DO Op
>                       |  JUMP Label
>                       |  JUMPZ Label
>                       |  LABEL Label
>                          deriving Show
>
> type Label            =  Int


Factorial example
-----------------

> fac                   :: Int -> Prog
> fac n                 =  Seqn [Assign 'A' (Val 1),
>                                Assign 'B' (Val n),
>                                While (Var 'B') (Seqn
>                                   [Assign 'A' (App Mul (Var 'A') (Var 'B')),
>                                    Assign 'B' (App Sub (Var 'B') (Val 1))])]