Iterate over data

work in progress, sorry

Clojure data is typically within one or more of the built in collection types (vector, map, list, set).

We can use some functions in Clojure core directly on these collection types. Other clojure core functions need a little help.


Used to create a new collection by applying a given function to each element of the collection in turn.

(map inc [1 2 3])

If there are multiple collections, map returns a new collection with values created by calling the function with a value from each of the collections. Once map reaches the end of one collection it stops and returns the result.

(map + [1 2 3] [4 5 6] [7 8 9])


Used to remove all the values from a collection so they are treated as individual arguemnts to the function given to apply.

(= (apply + [1 2 3])
   (+ 1 2 3))


reduce can be used in a similar way as apply, to transform a collection into a different value.

reduce can also take an argument referred to as an accumulator, used to keep local state as reduce iterates through the values in the collection.

A function used with reduce is called a reducing function and is a more abstract approach to loop/recur although its possible to give your reducing function a name so is more reusable.

threading macros

Write code that reads as a sequential series of function calls, rather that the nested function calls typical in lisp.

A threading macro is often used to thread a collection through a number of function calls and expressions.


Compose functions together that work over a collection. It can be seen as a more abstract approach to a threading macro or nested function calls.


Used like comp to create a pipeline of function calls, however, each function call or expression must return a transducer (transforming reduction). Many clojure.core functions return a transducer if you do not provide the collection argument.

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