## Stream: general

### Topic: Projection after mk-structure

#### Keeley Hoek (Sep 16 2018 at 16:39):

Say I've got a function mk_struct which takes some arguments and produces something of type struct. The function mk_struct needn't be the struct.mk constructor---maybe it takes some arguments, proves some stuff about them, and then packages it all up by calling the constructor struct.mk (the point is that the arguments of mk_struct can be totally different).

#### Keeley Hoek (Sep 16 2018 at 16:39):

Now say struct has a projection struct.foo. There is a single unambiguous way to construct a function mk_struct.foo, which forgets all of the components of whatever mk_struct outputs except for foo. I'd like to programmatically obtain the type of this function mk_struct.foo.

Actually, I've essentially done this, but it was incredibly stupid: I directly built everything over expr.xxx's and manually wrote code to unify the arguments of the projection and the output-type of mk_struct, and in the end it wasn't robust enough to deal with universe parameters.

#### Keeley Hoek (Sep 16 2018 at 16:40):

But I'm sure this was all crazy---I'm sure I can exploit lean's facilities (at the very least unify) to do the resolution which I need. Perhaps I'm looking for a suped-up tactic.mk_app that can unroll the quantifiers from the argument I want to apply? What's the canonical way to go about expr fiddling like this?

#### Mario Carneiro (Sep 16 2018 at 20:28):

I'm confused why this is different from struct.foo ∘ mk_struct. Can't you just figure out the type of this?

#### Keeley Hoek (Sep 17 2018 at 04:32):

@Mario Carneiro Thanks for taking a look. Here's a concrete example:

structure struct (n : ℤ) :=
(foo : ℕ)

def mk_struct (m : ℕ) : struct m := ⟨m, 2 * m⟩

#check struct.foo ∘ mk_struct
#check λ m, (mk_struct m).foo


The second-to-bottom line fails to typecheck, since mk_struct takes a (possibly a few) potentially random arguments. I'd really like to be able to compose to essentially do what the last line does, without having to know the arguments of mk_struct in advance.

#### Mario Carneiro (Sep 17 2018 at 04:45):

do you know the arguments of struct.foo?

#### Mario Carneiro (Sep 17 2018 at 04:46):

this function in tactic/alias.lean seems similar:

meta def mk_iff_mp_app (iffmp : name) : expr → (nat → expr) → tactic expr
| (expr.pi n bi e t) f := expr.lam n bi e <$> mk_iff_mp_app t (λ n, f (n+1) (expr.var n)) | (%%a ↔ %%b) f := pure$ @expr.const tt iffmp [] a b (f 0)
| _ f := fail "Target theorem must have the form Π x y z, a ↔ b"


it constructs the term \lam x y z, iff.mp (f x y z)`

Last updated: Dec 20 2023 at 11:08 UTC