Let's say I have a simple structure

structure foo : Type :=
  (a : ℕ)
  (b : list (fin a))

when defining the field b I can reference a. If I try to define a function to construct new foo's like so

def bar (p : foo) : foo :=
{ a := p.a + 1,
  b := list (fin a)

where I tried to reference the a I constructed on the first line, which gives me the error unknown identifier 'a'.

Is there a way to use previously constructed fields like this?

  • 2
    $\begingroup$ There seems to be a mistake in bar: going by the definition of foo, b is supposed to be a list of elements, not the type list (fin a) itself, right? $\endgroup$ Feb 17, 2022 at 17:31
  • $\begingroup$ Yes you're right, I just quickly constructed a simple example of what I was trying to illustrate what I wanted to do, that's why it doesn't make a lot of sense :) $\endgroup$
    – burek
    Feb 17, 2022 at 18:08

1 Answer 1


There isn't, and I don't think it would be a good idea to change that because it could easily lead to unintentional shadowing. The only language I know that has recursive record instantiation, Nix, has a dedicated syntax for that for presumably exactly that reason: rec { ... }. In Lean, you should use a let binding instead.


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