b5122b6a7b
This change * moves `termination_by` and `decreasing_by` next to the function they apply to * simplify the syntax of `termination_by` * apply the `decreasing_by` goal to all goals at once, for better interactive use. See the section in `RELEASES.md` for more details and migration advise. This is a hard breaking change, requiring developers to touch every `termination_by` in their code base. We decided to still do it as a hard-breaking change, because supporting both old and new syntax at the same time would be non-trivial, and not save that much. Moreover, this requires changes to some metaprograms that developers might have written, and supporting both syntaxes at the same time would make _their_ migration harder.
48 lines
1.4 KiB
Text
48 lines
1.4 KiB
Text
notation "#(" a ")" => ⟨a, by decreasing_tactic⟩
|
||
|
||
def List.unfoldr {α β : Type u} [sz : SizeOf β] (f : (b : β) → Option (α × { b' : β // sizeOf b' < sizeOf b})) (b : β) : List α :=
|
||
match f b with
|
||
| none => []
|
||
| some (a, ⟨b', h⟩) => a :: unfoldr f b'
|
||
|
||
def tst1 (n : Nat) : List Nat :=
|
||
List.unfoldr (b := n) fun
|
||
| 0 => none
|
||
| b+1 => some (3*n - b*2, #(b))
|
||
|
||
#eval tst1 10
|
||
|
||
def tst2 (n : Nat) : List Nat :=
|
||
-- Similar example where we provide our custom `SizeOf` instance
|
||
List.unfoldr (sz := ⟨fun b => n - b⟩) (b := 0) fun b =>
|
||
if h : b < n then
|
||
some (b*2, #(b+1))
|
||
else
|
||
none
|
||
|
||
#eval tst2 10
|
||
|
||
-- More general (and less convenient to use) version that can take an arbitrary well-founded relation.
|
||
def List.unfoldr' {α β : Type u} [w : WellFoundedRelation β] (f : (b : β) → Option (α × { b' : β // w.rel b' b})) (b : β) : List α :=
|
||
match f b with
|
||
| none => []
|
||
| some (a, ⟨b', h⟩) => a :: unfoldr' f b'
|
||
termination_by b
|
||
|
||
-- We need the `master` branch to test the following example
|
||
|
||
def tst3 (n : Nat) : List Nat :=
|
||
List.unfoldr' (b := n) fun
|
||
| 0 => none
|
||
| b+1 => some (3*n - b*2, #(b))
|
||
|
||
#eval tst3 10
|
||
|
||
def tst4 (n : Nat) : List Nat :=
|
||
List.unfoldr' (w := invImage (fun b => n - b) inferInstance) (b := 0) fun b =>
|
||
if h : b < n then
|
||
some (2*b, #(b+1))
|
||
else
|
||
none
|
||
|
||
#eval tst4 10
|