Error Reference › Lean 4
Lean 4
Unexpected token; expected term (Lean 4)
The parser hit something it did not expect at that position, so Lean could not even read the code; the message says what it expected instead.
What it looks like
Main.lean:5:0: error: unexpected token 'theorem'; expected termWhat does "Unexpected token; expected term (Lean 4)" mean?
This is a parse error, raised before any type checking. Lean prints unexpected token 'x' (or unexpected identifier), then expected and the things that could have come next. Here a definition ended with := and nothing after it, so the next command's keyword arrived where a term was expected. Parsing resumes at the next command, so errors inside the broken part only appear once this one is fixed.
What causes "Unexpected token; expected term (Lean 4)"?
- Lean 3 / mathlib3 syntax: begin … end, λ x, …, assume, or commas between tactics.
- A missing := or by, or a definition left unfinished before the next command.
- Indentation: a continuation line not indented, so the block ends early.
- A keyword (such as fun, at, from or end) used as a name, or unbalanced brackets.
How do I confirm the cause?
- Go to the line and column in the message; the problem is at that token or just before it.
- If the code came from a Lean 3 source (mathlib3, older tutorials), expect several of these — it needs porting, not patching.
How do I fix "Unexpected token; expected term (Lean 4)"?
- Look at the reported position and just before it — the real mistake is often on the previous line.
- Port Lean 3 syntax: fun x => … instead of λ x, …; := by with indented tactics instead of begin … end; one tactic per line or separated by ;.
- Finish the incomplete definition, or put sorry there for now.
- Check bracket pairs: ( ), ⟨ ⟩ and { }.
How do I stop it happening again?
- Use examples written for Lean 4 and current Mathlib.
- Fix parse errors as soon as the editor shows them; they hide other errors in the same declaration.