{-# OPTIONS --warning=error --safe --without-K #-}

open import LogicalFormulae

module Maybe where

data Maybe {a : _} (A : Set a) : Set a where
  no : Maybe A
  yes : A → Maybe A

joinMaybe : {a : _} → {A : Set a} → Maybe (Maybe A) → Maybe A
joinMaybe no = no
joinMaybe (yes s) = s

bindMaybe : {a b : _} → {A : Set a} → {B : Set b} → Maybe A → (A → Maybe B) → Maybe B
bindMaybe no f = no
bindMaybe (yes x) f = f x

applyMaybe : {a b : _} → {A : Set a} → {B : Set b} → Maybe (A → B) → Maybe A → Maybe B
applyMaybe f no = no
applyMaybe no (yes x) = no
applyMaybe (yes f) (yes x) = yes (f x)

yesInjective : {a : _} → {A : Set a} → {x y : A} → (yes x ≡ yes y) → x ≡ y
yesInjective {a} {A} {x} {.x} refl = refl

mapMaybe : {a b : _} → {A : Set a} → {B : Set b} → (f : A → B) → Maybe A → Maybe B
mapMaybe f no = no
mapMaybe f (yes x) = yes (f x)

defaultValue : {a : _} → {A : Set a} → (default : A) → Maybe A → A
defaultValue default no = default
defaultValue default (yes x) = x

noNotYes : {a : _} {A : Set a} {b : A} → (no ≡ yes b) → False
noNotYes ()

mapMaybePreservesNo : {a b : _} {A : Set a} {B : Set b} {f : A → B} {x : Maybe A} → mapMaybe f x ≡ no → x ≡ no
mapMaybePreservesNo {f = f} {no} pr = refl

mapMaybePreservesYes : {a b : _} {A : Set a} {B : Set b} {f : A → B} {x : Maybe A} {y : B} → mapMaybe f x ≡ yes y → Sg A (λ z → (x ≡ yes z) && (f z ≡ y))
mapMaybePreservesYes {f = f} {x} {y} map with x
mapMaybePreservesYes {f = f} {x} {y} map | yes z = z , (refl ,, yesInjective map)