------------------------------------------------------------------------
-- A monad-like structure
------------------------------------------------------------------------

{-# OPTIONS --without-K --safe #-}

module Delay-monad.Sized.Monad where

open import Prelude

open import Delay-monad.Sized
open import Delay-monad.Sized.Bisimilarity

------------------------------------------------------------------------
-- Monadic combinators

-- Return.

return :  {i a} {A : Size  Set a}  A i  Delay A i
return = now

-- Map. Note the function argument's type.

map :  {i a b} {A : Size  Set a} {B : Size  Set b} 
      ({j : Size< (ssuc i)}  A j  B j)  Delay A i  Delay B i
map f (now   x) = now (f x)
map f (later x) = later λ { .force  map f (force x) }

-- Join.

join :  {i a} {A : Size  Set a} 
       Delay (Delay A) i  Delay A i
join (now   x) = x
join (later x) = later λ { .force  join (force x) }

-- Bind. Note the function argument's type.

infixl 5 _>>=_

_>>=_ :  {i a b} {A : Size  Set a} {B : Size  Set b} 
        Delay A i  ({j : Size< (ssuc i)}  A j  Delay B j) 
        Delay B i
x >>= f = join (map f x)

------------------------------------------------------------------------
-- Monad laws

left-identity :
   {a b} {A : Size  Set a} {B : Size  Set b}
  x (f :  {i}  A i  Delay B i) 
  return x >>= f  f x
left-identity x f = reflexive (f x)

right-identity :  {a} {A : Size  Set a} (x : Delay A ) 
                 x >>= return  x
right-identity (now   x) = now
right-identity (later x) = later λ { .force 
                             right-identity (force x) }

associativity :
   {a b c} {A : Size  Set a} {B : Size  Set b} {C : Size  Set c} 
  (x : Delay A ) (f :  {i}  A i  Delay B i)
  (g :  {i}  B i  Delay C i) 
  x >>=  x  f x >>= g)  x >>= f >>= g
associativity (now   x) f g = reflexive (f x >>= g)
associativity (later x) f g = later λ { .force 
                                associativity (force x) f g }

------------------------------------------------------------------------
-- The functions map, join and _>>=_ preserve strong and weak
-- bisimilarity and expansion

map-cong :  {k i a b} {A : Size  Set a} {B : Size  Set b}
           (f :  {i}  A i  B i) {x y : Delay A } 
           [ i ] x  k  y  [ i ] map f x  k  map f y
map-cong f now        = now
map-cong f (later  p) = later λ { .force  map-cong f (force p) }
map-cong f (laterˡ p) = laterˡ (map-cong f p)
map-cong f (laterʳ p) = laterʳ (map-cong f p)

join-cong :  {k i a} {A : Size  Set a} {x y : Delay (Delay A) } 
              [ i ] x  k  y  [ i ] join x  k  join y
join-cong now        = reflexive _
join-cong (later  p) = later λ { .force  join-cong (force p) }
join-cong (laterˡ p) = laterˡ (join-cong p)
join-cong (laterʳ p) = laterʳ (join-cong p)

infixl 5 _>>=-cong_

_>>=-cong_ :
   {k i a b} {A : Size  Set a} {B : Size  Set b}
    {x y : Delay A } {f g :  {i}  A i  Delay B i} 
  [ i ] x  k  y  (∀ z  [ i ] f z  k  g z) 
  [ i ] x >>= f  k  y >>= g
now      >>=-cong q = q _
later  p >>=-cong q = later λ { .force  force p >>=-cong q }
laterˡ p >>=-cong q = laterˡ (p >>=-cong q)
laterʳ p >>=-cong q = laterʳ (p >>=-cong q)