gistfile1.scm

  ;; The first language in the middle end.
  (define-language M0
    (terminals
     (region-var (r))
     (variable (x name)))
    (Rho-Type
     (t)
     x)
    (Module
     (m)
     (module decl ...))
    (Decl
     (decl)
     (extern name (t* ...) -> t)
     (fn name (x ...) t body))
    (Body
     (body)
     (begin stmt ... body))
    (Stmt
     (stmt)
     (let ((x t e) ...) stmt))
    (Expr
     (e)
     (var t x)))

This gives me this error:

Exception in meta-parser: unrecognized meta variable -> in M0 at line 24, char 28 of harlan/middle/remove-lambdas.scm

Just to compete the thought here... (extern name (t* ...) -> t) is not a legal production in the current grammar, because literals are not allowed in the middle of a production (here the -> is treated as a meta-variable, but since it is not defined, the nanopass framework gets confused.)