# Introduction: Goals for this lab
+ Work with side effects and mutable data structures
## Answering the questions in this lab
Create a `lab12` directory in your team repository and copy the
`mutlist.ml` file to this directory; we'll work with the
contents of this file in the following section.
## Mutable lists
`mutlist.ml` defines the type `'a mlist`, a mutable list data
structure, and functions `mlist_of_list` and `list_of_mlist`
for converting between OCaml `list` values and `mlist` values.
In this problem, we'll write a few functions to manipulate mutable
lists; the equivalent immutable list versions already appear in
`mutlist.ml`:
+ `insert_after_m : 'a - 'a - 'a mlist - unit`: this function
performs a destructive update to its `mlist` argument: `insert_after_m
a b ml` should modify ml so that the value `b` appears after the
first instance of `a` in `ml`. For example:
```
# let m1 = mlist_of_list [2;3;5;11] ;;
val m1 : int mlist = C {hd = 2; tl = C {hd = 3; tl = C {hd = 5; tl = C {hd = 11; tl = Nil}}}}
# insert_after_m 5 7 ml;;
- : unit = ()
# list_of_mlist m1 ;;
- : int list = [2; 3; 5; 7; 11]
```
`insert_after_m` should raise `Not_found` if no instance of `b` is
encountered.
+ `exclude_m : ('a - bool) - 'a mlist - 'a mlist`: this function
destructively updates an mlist; `exclude_m p ml` should return a list that
excludes all elements of `ml` that satisfy the predicate `p`. For example:
```
# let m2 = mlist_of_list [1;2;3;4;5;6;7;8;9;10] ;;
# exclude_m (fun x - x mod 2 = 0) m2 ;;
- : int mlist = C {hd = 1; tl = C {hd = 3; tl = C {hd = 5; tl = C {hd = 7; tl = C {hd = 9; tl = Nil}}}}}
# list_of_mlist m2 ;;
- : int list = [1; 3; 5; 7; 9]
# exclude_m (fun x - x mod 3 = 1) m2 ;;
- : int mlist = C {hd = 3; tl = C {hd = 5; tl = C {hd = 9; tl = Nil}}}
# list_of_mlist m2 ;;
- : int list = [1; 3; 5; 9]
```
(Notice that only elements after the first element that does not satisfy `p`
are permanently removed from the list. Why? How could we change the
interface to `exclude_m` to prevent this problem?)
__This concludes lab 12.__
Note that any required changes must exist in your repository on
github.umn.edu. Doing the work but failing to push those changes
to your central repository will mean that we cannot see your work
and hence can't grade it.
