CMSC 22610
Implementation of Computer Languages - I
Winter 2009

Lab 1: Records, Datatypes, and Pattern Matching


Due: ∅
 

Welcome again! Datatypes and pattern matching are the champagne and caviar of ML programming. If you don't yet believe that, do this lab, and you will find your beliefs altered.

1  Files and Directories

Here is a datatype for a description of files and directories. It's recursive and shapely.

datatype file = F of {base: string, ext : string, size: int} (* size in bytes *) datatype file_or_dir = File of file | Dir of {name: string, contents: file_or_dir list}

Note that base and ext (for “extension”) are standard names for the parts of a filename before and after the dot, respectively. Here is an example file_or_dir value.

val mm = Dir {name="home", contents= [Dir {name="mork", contents= [File (F {base="strawberry-fields", ext="mp3", size=14144}), File (F {base="mindy", ext="jpg", size=2000})]}, Dir {name="mindy", contents= [File (F {base="mindy", ext="vcf", size=128}), Dir {name="pics", contents= [File (F {base="cat", ext="gif", size=1029}), File (F {base="dog", ext="png", size=555})]}]}]}

Write the functions corresponding to the following val specifications. Use pattern matching as liberally as possible but no more liberally. In particular, use underscores and record elisions (three dots) for values that need not be named. You should be able to infer what the following functions should compute by their names, types and comments.

val totalSize : file_or_dir -> int (* size in bytes *) val numFiles : file_or_dir -> int val dirNames : file_or_dir -> string list val images : file_or_dir -> file list val biggerThan : int -> file_or_dir -> file list val filesNamed : string -> file_or_dir -> file list val fullPaths : file_or_dir -> string list

The last function should evaluate on mm (from above) to the following:

["/home/mork/strawberry-fields.mp3", "/home/mork/mindy.jpg", "/home/mindy/mindy.vcf", "/home/mindy/pics/cat.gif", "/home/mindy/pics/dog.png"]

2  Errata

Revision 0 (Jan 14)

Last modified: Thu Feb 5 18:27:27 CST 2009