mirror of
https://github.com/edwinb/Idris2-boot.git
synced 2024-12-19 10:51:35 +03:00
04e4ebf80e
It's a big patch, but the summary is that it's okay to use a pattern in an erased position if either: - the pattern can also be solved by unification (this is the same as 'dot patterns' for matching on non-constructor forms) - the argument position is detaggable w.r.t. non-erased arguments, which means we can tell which pattern it is without pattern matching The second case, in particular, means we can still pattern match on proof terms which turn out to be irrelevant, especially Refl. Fixes #178
174 lines
5.0 KiB
Idris
174 lines
5.0 KiB
Idris
module Data.Fin
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import Data.Maybe
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import Data.Nat
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import Decidable.Equality
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||| Numbers strictly less than some bound. The name comes from "finite sets".
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||| It's probably not a good idea to use `Fin` for arithmetic, and they will be
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||| exceedingly inefficient at run time.
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||| @ n the upper bound
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public export
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data Fin : (n : Nat) -> Type where
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FZ : Fin (S k)
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FS : Fin k -> Fin (S k)
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export
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implementation Uninhabited (Fin Z) where
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uninhabited FZ impossible
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uninhabited (FS f) impossible
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export
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FSInjective : (m : Fin k) -> (n : Fin k) -> FS m = FS n -> m = n
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FSInjective left _ Refl = Refl
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export
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implementation Eq (Fin n) where
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(==) FZ FZ = True
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(==) (FS k) (FS k') = k == k'
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(==) _ _ = False
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||| There are no elements of `Fin Z`
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export
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FinZAbsurd : Fin Z -> Void
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FinZAbsurd FZ impossible
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export
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FinZElim : Fin Z -> a
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FinZElim x = void (FinZAbsurd x)
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||| Convert a Fin to a Nat
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public export
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finToNat : Fin n -> Nat
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finToNat FZ = Z
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finToNat (FS k) = S (finToNat k)
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||| `finToNat` is injective
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export
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finToNatInjective : (fm : Fin k) -> (fn : Fin k) -> (finToNat fm) = (finToNat fn) -> fm = fn
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finToNatInjective (FS m) FZ Refl impossible
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finToNatInjective FZ (FS n) Refl impossible
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finToNatInjective (FS m) (FS n) prf =
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cong FS (finToNatInjective m n (succInjective (finToNat m) (finToNat n) prf))
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finToNatInjective FZ FZ Refl = Refl
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export
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implementation Cast (Fin n) Nat where
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cast x = finToNat x
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||| Convert a Fin to an Integer
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public export
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finToInteger : Fin n -> Integer
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finToInteger FZ = 0
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finToInteger (FS k) = 1 + finToInteger k
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export
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implementation Cast (Fin n) Integer where
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cast x = finToInteger x
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||| Weaken the bound on a Fin by 1
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public export
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weaken : Fin n -> Fin (S n)
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weaken FZ = FZ
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weaken (FS k) = FS (weaken k)
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||| Weaken the bound on a Fin by some amount
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public export
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weakenN : (n : Nat) -> Fin m -> Fin (m + n)
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weakenN n FZ = FZ
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weakenN n (FS f) = FS (weakenN n f)
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||| Attempt to tighten the bound on a Fin.
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||| Return `Left` if the bound could not be tightened, or `Right` if it could.
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export
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strengthen : {n : _} -> Fin (S n) -> Either (Fin (S n)) (Fin n)
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strengthen {n = S k} FZ = Right FZ
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strengthen {n = S k} (FS i) with (strengthen i)
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strengthen (FS i) | Left x = Left (FS x)
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strengthen (FS i) | Right x = Right (FS x)
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strengthen f = Left f
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||| Add some natural number to a Fin, extending the bound accordingly
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||| @ n the previous bound
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||| @ m the number to increase the Fin by
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public export
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shift : (m : Nat) -> Fin n -> Fin (m + n)
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shift Z f = f
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shift {n=n} (S m) f = FS {k = (m + n)} (shift m f)
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||| The largest element of some Fin type
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public export
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last : {n : _} -> Fin (S n)
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last {n=Z} = FZ
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last {n=S _} = FS last
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export total
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FSinjective : {f : Fin n} -> {f' : Fin n} -> (FS f = FS f') -> f = f'
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FSinjective Refl = Refl
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export
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implementation Ord (Fin n) where
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compare FZ FZ = EQ
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compare FZ (FS _) = LT
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compare (FS _) FZ = GT
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compare (FS x) (FS y) = compare x y
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-- Construct a Fin from an integer literal which must fit in the given Fin
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public export
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natToFin : Nat -> (n : Nat) -> Maybe (Fin n)
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natToFin Z (S j) = Just FZ
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natToFin (S k) (S j)
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= case natToFin k j of
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Just k' => Just (FS k')
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Nothing => Nothing
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natToFin _ _ = Nothing
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||| Convert an `Integer` to a `Fin`, provided the integer is within bounds.
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||| @n The upper bound of the Fin
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public export
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integerToFin : Integer -> (n : Nat) -> Maybe (Fin n)
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integerToFin x Z = Nothing -- make sure 'n' is concrete, to save reduction!
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integerToFin x n = if x >= 0 then natToFin (fromInteger x) n else Nothing
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||| Allow overloading of Integer literals for Fin.
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||| @ x the Integer that the user typed
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||| @ prf an automatically-constructed proof that `x` is in bounds
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public export
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fromInteger : (x : Integer) -> {n : Nat} ->
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{auto prf : (IsJust (integerToFin x n))} ->
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Fin n
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fromInteger {n} x {prf} with (integerToFin x n)
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fromInteger {n} x {prf = ItIsJust} | Just y = y
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||| Convert an Integer to a Fin in the required bounds/
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||| This is essentially a composition of `mod` and `fromInteger`
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public export
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restrict : (n : Nat) -> Integer -> Fin (S n)
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restrict n val = let val' = assert_total (abs (mod val (cast (S n)))) in
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-- reasoning about primitives, so we need the
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-- 'believe_me'. It's fine because val' must be
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-- in the right range
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fromInteger {n = S n} val'
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{prf = believe_me {a=IsJust (Just val')} ItIsJust}
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--------------------------------------------------------------------------------
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-- DecEq
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--------------------------------------------------------------------------------
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export total
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FZNotFS : {f : Fin n} -> FZ {k = n} = FS f -> Void
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FZNotFS Refl impossible
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export
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implementation DecEq (Fin n) where
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decEq FZ FZ = Yes Refl
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decEq FZ (FS f) = No FZNotFS
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decEq (FS f) FZ = No $ negEqSym FZNotFS
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decEq (FS f) (FS f')
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= case decEq f f' of
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Yes p => Yes $ cong FS p
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No p => No $ \h => p $ FSinjective {f = f} {f' = f'} h
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