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Editorial Team
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Asked: 2026-05-27T15:45:56+00:00

At the Clojure REPL, this expression ( #(for [x %] (+ 100 (second x)))

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At the Clojure REPL, this expression

( #(for [x %] (+ 100 (second x)))  ['(+ 38) '(+ 48)] )

produces (138 148) as expected

but this

( #(for [x %] ((first x) 100 (second x))) ['(+ 38) '(+ 48)] )

produces (38 48) which seems truly weird.

Both expressions really should be producing the same result!
What am I missing?
Will appreciate any ideas to resolve this mystery.

BTW, I tried to use ‘apply (first x)’ and package the rest of the args into a list but it doesn’t seem to matter. The same unexpected result comes back.

Also, to verify that the + indeed gets resolved from the input, I gave the following to the REPL

( #(for [x %] (resolve (first x) )) '((+ 38) (+ 48)) )

which produced

 (#'clojure.core/+ #'clojure.core/+)   as expected.
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1 Answer

  1. Editorial Team
    ( #(for [x %] ((first x) 100 (second x))) ['(+ 38) '(+ 48)] )

    In this the + is a symbol, not a function, because it has been quoted in the list. However, symbols are defined as doing map lookup when invoked as a function (the same as keywords). So ('+ 100 38) is the same as (get 100 '+ 38). That last argument is the “if you can’t find what I want in the map, return that”. Since 100 is not a map, + uses that argument as the return value.

    To make it do what you want you have two options:

    1. Use vectors instead of quoted lists ensures that + gets resolved appropriately.

      ( #(for [x %] ((first x) 100 (second x))) [[+ 38] [+ 48]] )

    2. Resolve it yourself to ensure that you use the + function instead of the + symbol.

      ( #(for [x %] ((resolve (first x)) 100 (second x))) ['(+ 38) '(+ 48)] )
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