113 lines
3.1 KiB
Text
113 lines
3.1 KiB
Text
/-
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Copyright (c) 2020 Microsoft Corporation. All rights reserved.
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Released under Apache 2.0 license as described in the file LICENSE.
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Authors: Sebastian Ullrich, Andrew Kent, Leonardo de Moura
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-/
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prelude
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import Init.Data.Array.Subarray
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import Init.Data.Range
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/-
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Remark: we considered using the following alternative design
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```
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structure Stream (α : Type u) where
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stream : Type u
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next? : stream → Option (α × stream)
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class ToStream (collection : Type u) (value : outParam (Type v)) where
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toStream : collection → Stream value
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```
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where `Stream` is not a class, and its state is encapsulated.
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The key problem is that the type `Stream α` "lives" in a universe higher than `α`.
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This is a problem because we want to use `Stream`s in monadic code.
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-/
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/-
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Streams are used to implement parallel `for` statements.
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Example:
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```
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for x in xs, y in ys do
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...
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```
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is expanded into
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```
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let mut s := toStream ys
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for x in xs do
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match Stream.next? s with
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| none => break
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| some (y, s') =>
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s := s'
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...
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```
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-/
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class ToStream (collection : Type u) (stream : outParam (Type u)) : Type u where
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toStream : collection → stream
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export ToStream (toStream)
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class Stream (stream : Type u) (value : outParam (Type v)) : Type (max u v) where
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next? : stream → Option (value × stream)
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protected partial def Stream.forIn [Stream ρ α] [Monad m] (s : ρ) (b : β) (f : α → β → m (ForInStep β)) : m β := do
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let inst : Inhabited (m β) := ⟨pure b⟩
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let rec visit (s : ρ) (b : β) : m β := do
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match Stream.next? s with
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| some (a, s) => match (← f a b) with
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| ForInStep.done b => return b
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| ForInStep.yield b => visit s b
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| none => return b
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visit s b
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instance (priority := low) [Stream ρ α] : ForIn m ρ α where
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forIn := Stream.forIn
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instance : ToStream (List α) (List α) where
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toStream c := c
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instance : ToStream (Array α) (Subarray α) where
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toStream a := a[:a.size]
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instance : ToStream (Subarray α) (Subarray α) where
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toStream a := a
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instance : ToStream String Substring where
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toStream s := s.toSubstring
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instance : ToStream Std.Range Std.Range where
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toStream r := r
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instance [Stream ρ α] [Stream γ β] : Stream (ρ × γ) (α × β) where
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next?
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| (s₁, s₂) =>
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match Stream.next? s₁ with
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| none => none
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| some (a, s₁) => match Stream.next? s₂ with
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| none => none
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| some (b, s₂) => some ((a, b), (s₁, s₂))
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instance : Stream (List α) α where
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next?
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| [] => none
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| a::as => some (a, as)
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instance : Stream (Subarray α) α where
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next? s :=
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if h : s.start < s.stop then
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have : s.start + 1 ≤ s.stop := Nat.succ_le_of_lt h
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some (s.as.get ⟨s.start, Nat.lt_of_lt_of_le h s.h₂⟩, { s with start := s.start + 1, h₁ := this })
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else
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none
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instance : Stream Std.Range Nat where
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next? r :=
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if r.start < r.stop then
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some (r.start, { r with start := r.start + r.step })
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else
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none
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instance : Stream Substring Char where
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next? s :=
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if s.startPos < s.stopPos then
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some (s.str.get s.startPos, { s with startPos := s.str.next s.startPos })
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else
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none
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