Back to Contents

The OCaml Standard Library

OCaml is provided with a wide range of useful built-in functions, in addition to the ones we have already seen, called the OCaml Standard Library. These functions are divided into modules, one for each area of functionality (in the next chapter, we will learn how to write our own modules). Here are a few examples of modules in the standard library:


We will take the List module as an example. You can find the documentation for the OCaml Standard Library installed with your copy of OCaml, or on the internet.

The functions from a module can be used by putting a period (full stop) between the module name and the function. For example the length function in the List module can be used like this:


# List.length [1; 2; 3; 4; 5];;
- : int = 5

We can look at the type too by writing just the name of the function:


# List.length;;
- : 'a list -> int = <fun>

Here’s the documentation for List.length:


We will talk about val in the next chapter. Sometimes, more information is required:


For example,


# List.nth [1; 2; 4; 8; 16] 3;;
- : int = 8

In the documentation, we are told what the function does for each argument, and what exceptions may be raised. Functions which are not tail-recursive and so may fail on huge arguments are marked as such.

The questions for this chapter use functions from the standard library, so you will need to have a copy of the documentation to hand.


  1. Write your own version of the function List.concat. The implementation OCaml provides is not tail-recursive. Can you write one which is?

  2. Use List.mem to write a function which returns true only if every list in a bool list list contains true somewhere in it.

  3. Write a function to count the number of exclamation marks in a string, using one or more functions from the String module.

  4. Use the function to write a function to return a new copy of a string with all exclamation marks replaced with periods (full stops).

  5. Use the String module to write a function which concatenates a list of strings together.

  6. Do the same with the Buffer module. This will be faster.

  7. Use the String module to count the number of occurrences of the string "OCaml" within a given string.