specifying `:let` as the first element of `for` causes an macro expansion error

Question

We can use :let in for to define local variable like this

(for [i (range 5)
      :let [inc-i (inc i)]]
  [i inc-i])
;;=> ([0 1] [1 2] [2 3] [3 4] [4 5])

Otherwise, if we specified :let as first element, it causes an macro expansion error.

(for [:let [range-end 5]
      i (range range-end)
      :let [inc-i (inc i)]]
  [i inc-i])
;;=> Syntax error macroexpanding for at (*cider-scratch*:555:1).
;;   Can't pop empty vector

Of course, in this case, we should have wrapped for in let. However, I'm curious whether making :let available only for second and subsequent elements is intended behavior as part of the public interface.

Answer 1

Seems like it is working as intended per the docstring.

Comments

Agreed:

"Takes a vector of one or more binding-form/collection-expr pairs, each followed by zero or more modifiers..."

:let is a modifier -- it has to be preceded by a binding-form/collection-expr pair.

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Thanks both! I finally understand.

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For beginners, couldn't a more informative error message be displayed?

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Possibly, but the extra check on the vector would be a performance penalty that everyone would pay for every use of `for` in every program -- and Clojure rarely does that to its users.