How to make list of partial functions in Clojure?

Question

i need to write code like this in clojure.

-- haskell
fns = map (,) [1..3]
head fns $ 1
-- => (1,1)
fns <*> [1..3]
-- => [(1,1),(1,2),(1,3),(2,1),(2,2),(2,3),(3,1),(3,2),(3,3)]

doesn't work

(def fns (map (partial list) (range 1 3)))
((first fns) 1)
;; => ClassCastException java.lang.Long cannot be cast to clojure.lang.IFn

works, but i think this isn't idiomatic way

(def fns (map (fn [x] `(partial list ~x)) (range 1 3)))
((eval (first fns)) 1)
;; => (1 1)

Did you look at what the ClassCastException is telling you? — Robert Harvey
– Robert Harvey, Commented Jun 1, 2014 at 21:13
Yes. So, I wrote the last code. But, I think that code is ugly. How can I make this better? — user3697543
– user3697543, Commented Jun 1, 2014 at 21:18
OK, but rather than punting and using eval, is there any way to make the cast succeed? What about this: stackoverflow.com/q/14559071 — Robert Harvey
– Robert Harvey, Commented Jun 1, 2014 at 21:18
How can I take a function from list without evaluating. And is there any way to use that function without eval? — user3697543
– user3697543, Commented Jun 1, 2014 at 21:36

amalloy · Accepted Answer · 2014-06-01 23:19:28Z

6

The function (partial list) is equivalent to just the function list. It's not like haskell where everything is curried - I think you intended partial to see that it's been given only one argument, list, and curry itself up to wait for a second argument. But really that should be (partial partial list): you are attempting to partially apply the function partial itself.

Note also that partially-applied functions are not as common in clojure as they are in haskell, partly because they just don't read so well. Instead of (map (partial partial list) (range 1 3)), if I wanted to build a list of functions like this, I would probably write (for [i (range 1 3)] (fn [j] (list i j))).

answered Jun 1, 2014 at 23:19

amalloy

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