Get­ting start­ed with Lean 4, your next pro­gram­ming lan­guage

Need­ed tools

I‘m on ma­cOS and use Home­brew ex­ten­sive­ly. My sim­ple go-to ap­proach to find­ing new soft­ware is to do brew search lean. This re­vealed lean as well as sur­face elan. Run­ning brew info lean showed me that that pack­age (at the time I write this) in­stalls Lean 3. But I know, out-of-band, that Lean 4 is what I want to work with. Run­ning brew info elan looked bet­ter, but the out­put re­minds me that (1) the in­for­ma­tion is for the elan-init pack­age, not the elan cask, and (2) elan-init con­flicts with both the elan and the afore­men­tioned lean. Yikes! This strikes me as a po­ten­tial prob­lem for the com­mu­ni­ty, be­cause I think Lean 3, though it still works, is pre­sum­ably not where new Lean de­vel­op­ment should be tak­ing place. Per­haps the Home­brew for­mu­la for Lean should be up­dat­ed called lean3, and a new lean4 pack­age should be made avail­able. I‘m not sure. The sit­u­a­tion seems less than ide­al, but in short, I have been suc­cess­ful with the elan-init pack­age.

Af­ter in­stalling elan-init, you‘ll have the elan tool avail­able in your shell. elan is the tool used for main­tain­ing dif­fer­ent ver­sions of Lean, sim­i­lar to nvm in the Node.js world or pyenv.

Set­ting up a blank pack­age

When I did the Lean 4 tu­to­r­i­al at BOB, I worked en­tire­ly with­in VS Code and cre­at­ed a new stand­alone pack­age us­ing some in-ed­i­tor func­tion­al­i­ty. At the com­mand line, I use lake init to man­u­al­ly cre­ate a new Lean pack­age. At first, I made the mis­take of run­ning this com­mand, as­sum­ing it would cre­ate a new di­rec­to­ry for me and set up any con­fig­u­ra­tion and boil­er­plate code there. I was sur­prised to find, in­stead, that lake init sets things up in the cur­rent di­rec­to­ry, in ad­di­tion to cre­at­ing a sub­di­rec­to­ry and pop­u­lat­ing it. Us­ing lake --help, I read about the lake new com­mand, which does what I had in mind. So I might sug­gest us­ing lake new rather than lake init.

What‘s in the new di­rec­to­ry? Do­ing tree foo­bar re­veals

foo­bar
├── Foo­bar
│   └── Ba­sic.lean
├── Foo­bar.lean
├── Main.lean
├── lake­file.lean
└── lean-tool­chain

Tak­ing a look there, I see four .lean files. Here‘s what they con­tain:

im­port «Foo­bar»

def main : IO Unit :=
  IO.print­ln s!"Hel­lo, {hel­lo}!"

-- This mod­ule serves as the root of the `Foo­bar` li­brary.
-- Im­port mod­ules here that should be built as part of the li­brary.
im­port «Foo­bar».Ba­sic

def hel­lo := "world"

im­port Lake
open Lake DSL

pack­age «foo­bar» where
  -- add pack­age con­fig­u­ra­tion op­tions here

lean_lib «Foo­bar» where
  -- add li­brary con­fig­u­ra­tion op­tions here

@[de­fault_tar­get]
lean_exe «foo­bar» where
  root := `Main

It looks like there‘s a lit­tle mod­ule struc­ture here, and a ref­er­ence to the iden­ti­fi­er hel­lo, de­fined in Foo­bar/Ba­sic.lean and made avail­able via Foo­bar.lean. I’m not go­ing to touch lake­file.lean for now; as a new­bie, it looks scary enough that I think I’ll just stick to things like Ba­sic.lean.

There‘s also an au­to­mat­i­cal­ly cre­at­ed .git there, not shown in the di­rec­to­ry out­put above.

Now what?

Now that you‘ve got Lean 4 in­stalled and set up a pack­age, you‘re ready to dive in to one of the of­fi­cial tu­to­ri­als. The one I‘m work­ing through is David‘s Func­tion­al Pro­gram­ming in Lean. There‘s all sorts of ad­di­tion­al things to learn, such as all the dif­fer­ent lake com­mands. En­joy!