Types.hs

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{-# LANGUAGE GADTs #-}


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module Console.Garepinoh.Types
    ( Result
    , Prelude
    , Stack
    , El(..)
    , NonEmptyList(..)
    , Func(..)
    , Function(..)
    ) where


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-- |'Result' is either an error message or a 'Stack'.
type Result t = Either String (Stack t)


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-- |A 'Prelude' is a list of pre-defined functions.
type Prelude t = [Func t]


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-- |A 'Stack' is (simulated by) a list of elements.
type Stack t = [El t]


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-- |An element is either...
data El t = Va t        -- ^ a value of type @t@,
          | Li [El t]   -- ^ a list of elements, or
          | Fu (Func t) -- ^ a function

instance Eq t => Eq (El t) where
    Va a == Va b = a == b
    Li a == Li b = a == b
    Fu a == Fu b = hd (symb a) == hd (symb b)
    _ == _ = False

instance Show t => Show (El t) where
    show (Va n) = show n
    show (Li l) = show l
    show (Fu f) = show f


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-- |'NonEmptyList' is a non-empty list.
data NonEmptyList t = NEL
    { hd :: t   -- ^ first element  (head) of the (non-empty) list.
    , tl :: [t] -- ^ remaining part (tail) of the (non-empty) list.
    }


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-- |A Function consists of:
data Func t = Func
    { symb :: NonEmptyList String
      -- ^ a non-empty list of identifiers;
    , func :: [Function t]
      -- ^ a list (@[f_fst,f_snd,..,f_last@]) of
      -- primitive/undivisible 'Function's which are evaluated step-by-step.
    }

instance Show (Func t) where show = hd . symb


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-- |A primitive/undivisible 'Function' is one of:
      -- 
      -- 
data Function t
    = Ele (El t)
      -- ^ an element 'El';
    | Ref String
      -- ^ a reference to other functions; or
    | Fun (Prelude t -> Stack t -> Either String (Stack t))
      -- ^ an actual primitive/pre-defined/hard-coded function.