Overview
The purpose of this assignment is to help you get acclimated to programming in Standard ML, which you will use in the next few weeks to implement type systems and lambda calculus. The assignment has three parts:
 To begin, you will answer some questions about reading.
 On your own, you will write many small exercises.
 Possibly working with a partner, you will make a small change to the μScheme interpreter that is written in ML (in Chapter 5).
By the time you complete this assignment, you will be ready to tackle serious programming tasks in Standard ML.
Prelude
Setup
COMP 105 uses two different implementations of Standard ML. For the small problems, we recommend Moscow ML, which is in /usr/sup/bin/mosml
. To start Moscow ML, use
ledit mosml P full
If everything is working correctly, you should see this prompt:
Moscow ML version 2.103 (Tufts University, April 2012)
Enter `quit();' to quit.

If you don’t see the Tufts name, send an immediate email of complaint to staff@cs.tufts.edu
, with a copy to comp105grades@cs.tufts.edu
.
For the large problem, we recommend a nativecode compiler called mlton
(pronounced “Milton”).
The initial basis
As in the hofs
assignment, we expect you to use the initial basis, which is properly known as the Standard ML Basis Library. By the standards of popular languages, the basis is quite small, but it is still much more than you can learn in a week. Fortunately, you only have to learn a few key parts:
 Type constructors
list
,option
,bool
,int
,string
, andorder
 Modules
List
andOption
, includingList.filter
,List.exists
,List.find
, and others  Other module functions
Int.toString
,Int.compare
, andString.compare
 Toplevel functions
o
,print
(for debugging),map
,app
,foldr
,foldl
The most convenient guide to the basis is the Moscow ML help system; type
 help "";
at the mosml
interactive prompt. The help file is badged incorrectly, but as far as I know, it is up to date.
If you have Jeff Ullman’s text, you need to know that Chapter 9 describes the 1997 basis, which is out of date: today’s compilers use the 2004 basis, which is a standard. But there are only a few differences, primarily in I/O and arrays. The most salient difference is in the interface to TextIO.inputLine
.
Things you need to review before starting
We provide a guide to Learning Standard ML. And pages 858–861 in Ramsey’s book contain a large table that may help you translate μScheme code into core ML. Skim these materials before starting, so you know what is there. Learning Standard ML will guide you to other reading.
How to develop an acceptable style
Learning Standard ML refers to you books by Ullman, Ramsey, and Harper, and to a technical report by Tofte. Ullman provides the most gentle introduction to ML, and he provides the most information about ML. His book is especially good for programmers whose primary experience is in Clike languages. But, to put it politely, Ullman’s ML is not idiomatic. Much of what you see in Ullman should not be imitated. The code by Ramsey, starting in Chapter 5, is a better guide to what ML should look like.
Harper’s code is also very good, and Tofte’s code is reasonable.
I recommend that you focus on getting your code working first. Then submit it. Then pick up our “Style Guide for Standard ML Programmers”, which contains many examples of good and bad style. Edit your code lightly to conform to the style guide, and submit it again.
In the long run, we expect you to master and follow the guidelines in the style guide.
Type checking versus unit testing
Standard ML is a language designed for production, not teaching. It has no checkexpect
or other unittesting support. It does, however, support some checking of types. In particular, you can check the type of an identifier by rebinding the identifier using an explicit type. There are examples below, and you can run them against your code by running
% mlsanitycheck warmup.sml
You can also whip up a poor substitute for unit testing using this code as a model:
exception AssertionFailure
fun assert p = if p then () else raise AssertionFailure
val _ = assert(length ["a", "b", "c"] = 3)
val _ = assert(getOpt (NONE, 99) = 99)
val _ = assert(getOpt (SOME 12, 99) = 12)
Not great, but better than nothing.
Dire warnings
There some functions and idioms that you must avoid. Code violating any of these guidelines will earn No Credit.
When programming with lists, it is rarely necessary or desirable to use the
length
function. The entire assignment can and should be solved without usinglength
.Solutions that use
length
will earn No Credit.Use function definition by pattern matching. Do not use the functions
null
,hd
, andtl
; use patterns instead.Solutions that use
hd
ortl
will earn No Credit.Do not define auxiliary functions at top level. Use
local
orlet
.Solutions that define auxiliary functions at top level will earn No Credit.
Do not use
open
; if needed, use short abbreviations for common structures. For example, if you want frequent access to theListPair
structure, you can writestructure LP = ListPair
and from there on you can refer to, e.g.,
LP.map
.Solutions that use
open
may earn No Credit for your entire assignment.Unless the problem explicitly says it is OK, do not use any imperative features.
Unless explicitly exempted, solutions that use imperative features will earn No Credit.
Reading comprehension (10%)
These problems will help guide you through the reading. We recommend that you complete them before starting the other problems below. You can download the questions.
Read section 5.1 of Harper about tuple types and tuple patterns. Also look at the list examples in sections 9.1 and 9.2 of Harper.
Now consider the pattern
(x::y::zs, w)
. For each of the following expressions, tell whether the pattern matches the value denoted. If the pattern matches, say what values are bound to the four variablesx
,y
,zs
, andw
. If it does not match, explain why not.([1, 2, 3], ("COMP", 105))
(("COMP", 105), [1, 2, 3])
([("COMP", 105)], (1, 2, 3))
(["COMP", "105"], true)
([true, false], 2.718281828)
Answers here:
You are now starting to be ready to use pattern matching.
In Standard ML, string literals are what you’re used to from C and C++, but character literals are not. Consult page 21 of Harper or page 13 of Ullman and answer these questions:
What string literal is used to write the string “hello” of type
string
?→
What character literal is used to write the capital A character, of type
char
?→
What character literal is used to write the newline character, of type
char
?→
You are now ready to match patterns in the “first vowel” problem (D).
Look at the clausal function definition of
outranks
on page 83 of Harper. Using the clausal definition enables us to avoid nestedcase
expressions such as we might find in Standard ML or μML, and it enables us to avoid nestedif
expressions such as we might find in μScheme. This particular example also collapses multiple cases by using the “wildcard pattern”_
.A wildcard by itself can match anything, but a wildcard in a clausal definition can match only things that are not matched by preceding clauses. Answer these questions about the wildcards in
outranks
:In the second clause, what three suits can the
_
match?→
In the fifth clause, what suits can the
_
match?→
In the eighth and final clause, what suits can the
_
match?→
You are now ready to match patterns that combine tuples with algebraic data types.
In Ramsey’s chapter 5, the
eval
code for applying a function appears in code chunk 357c. In evaluatingAPPLY (f, args)
, if expressionf
does not evaluate to either a primitive function or a closure, the code raises theRuntimeError
exception.Show a piece of μScheme code that would, when evaluated, cause chunk 357c to raise the
RuntimeError
exception.→
When exception
RuntimeError
is raised, what happens from the user’s point of view?Suppose the line that raises the exception were removed from the interpreter. If the modified interpreter then tried to evaluate the μScheme code from part (a), what would happen from the user’s point of view?
You are now ready for problems G, L, and M.
We discussed “free” variables briefly in class. You can find a longer discussion and precise definition in section 5.10 of Ramsey’s book, which starts on page 368. Read the section and identify the free variables of the following expressions:
Free variables of
(lambda (x) (lambda (y) (equal? x y)))
→
Free variables of
(lambda (y) (equal? x y))
→
Free variables of
(lambda (s1 s2) (if (or (atom? s1) (atom? s2)) (= s1 s2) (and (equal? (car s1) (car s2)) (equal? (cdr s1) (cdr s2)))))
→
You are now ready to improve the μScheme interpreter in problem 2.
Programming problems to solve individually (75%)
How to organize your code
Most of your solutions will go into a single file: warmup.sml
. But in problem M, you’ll implement environments in two different ways, and you’ll write code that works with both implementations. Those solutions will go in files envdata.sml
, envfun.sml
, and envboth.sml
. You’ll also want to download envdatatest.sml
and envfuntest.sml
.
At the start of each problem, please label it with a short comment, like
(***** Problem A *****)
To receive credit, your warmup.sml
file must compile and execute in the Moscow ML system. For example, we must be able to compile your code without warnings or errors. The following three commands should test all of your code:
% /usr/sup/bin/mosmlc c warmup.sml
% testenvdata
% testenvcode
Please remember to put your name and the time you spent in the warmup.sml
file.
The problems
Working on your own, please solve the following problems:
Defining functions using clauses and patterns
Related Reading for problems A to D: In Learning Standard ML read about Expressions (sections I, II, and III), Data (I, II, and II), Inexhaustive pattern matches, Types (I), Definitions (III, IV), and Expressions (VIII).
A. Write the function null
, which when applied to a list tells whether the list is empty. Avoid if
, and make sure the function takes constant time. Make sure your function has the same type as the null
in the Standard Basis.
You are welcome to translate the μML null?
function defined in Ramsey, chunk 756a.
B. Chunk 772c of Ramsey’s book contains a clausal definition of a μML function samelength?
. In Standard ML, such a function would be called sameLength
and would have type
val sameLength : 'a list * 'b list > bool
Using a clausal definition (fun
without case
), translate μML function samelength?
into Standard ML function sameLength
. Do not use if
or case
.
C. Using patterns, write a recursive Fibonacci function that does not use if
. Calling fib n
should return the n
th Fibonacci number, e.g,. fib 0
is 0, fib 1
is 1, fib 4
is 3. Taking exponential time is OK.
D. Write a function firstVowel
that takes a list of lowercase letters and returns true
if the first character is a vowel (aeiou) and false
if the first character is not a vowel or if the list is empty. Use the wildcard symbol _
whenever possible, and avoid if
.
Lists
Related Reading for problems E to J: In Learning Standard ML, apart from
the section noted above, read about Types (III), and Exceptions. For this section, you will need to understand lists and pattern matching on lists well (see Data III). You may also wish to read the section on Curried Functions.
E. Functions foldl
and foldr
are predefined with type
('a * 'b > 'b) > 'b > 'a list > 'b
They are like the μScheme versions except the ML versions are Curried.
Implement
rev
(the function known in μScheme asreverse
) usingfoldl
orfoldr
.Implement
minlist
, which returns the smallest element of a nonempty list of integers. Usefoldl
orfoldr
.If given an empty list of integers, your solution can fail (e.g., by
raise Match
).Your solution should work regardless of the representation of integers: it should not matter how many bits are used to represent a value of type
int
. (Hint: The course solutionmax*
from the hofs homework works regardless of the representation of integers. Perhaps you can steal an idea from it.)
Do not use recursion in either part of this problem.
F. Implement foldl
and foldr
using recursion. Do not create unnecessary cons cells. Do not use if
.
You are welcome to translate the μML code from chunk 1379h of Ramsey’s book—just be aware that the μML version is not curried, and the version you write must be curried.
G. Write a function zip: 'a list * 'b list > ('a * 'b) list
that takes a pair of lists (of equal length) and returns the equivalent list of pairs. If the lengths don’t match, raise the exception Mismatch
, which you will have to define.
You are welcome to translate a solution from μScheme, but you must either use a clausal definition or write code containing at most one case
expression. Do not use if
.
H. Define a function
val pairfoldr : ('a * 'b * 'c > 'c) > 'c > 'a list * 'b list > 'c
that applies a threeargument function to a pair of lists of equal length, using the same order as foldr
.
Use pairfoldr
to implement zip
.
I. Define a function
val unzip : ('a * 'b) list > 'a list * 'b list
that turns a list of pairs into a pair of lists. Do not use if
.
This one is tricky; here’s a sample result:
 unzip [(1, true), (3, false)];
> val it = ([1, 3], [true, false]) : int list * bool list
You are welcome to translate any of the solutions from μScheme, but you may not use if
.
J. Define a function
val concat : 'a list list > 'a list
which takes a list of lists and produces a single list containing all the elements in the correct order. For example,
 concat [[1], [2, 3, 4], [], [5, 6]];
> val it = [1, 2, 3, 4, 5, 6] : int list
Do not use if
. Do not simply call List.concat
—you will earn No Credit.
To get full credit for this problem, your function should use no unnecessary cons cells.
Note: Function concat
is related to the flatten
that you implemented in μScheme, but concat
“goes only one level down.”
Higherorder programming
Related Reading for problem K: The reading for the Lists problems should guide you in this section as well.
K. Function compound
is something like a fold
, but it operates on a restricted class of functions: the first argument to compound
is a function of type 'a * 'a > 'a
, which means it takes two arguments of the same type and returns a result also of that type. Examples of such functions include functions like op +
and op *
, but not op ::
(cons). Every function that can be used with compound
can be used with foldr
, but not vice versa.
Function compound
has this type:
val compound : ('a * 'a > 'a) > int > 'a > 'a
and compound f n x
is
x
ifn=0
,f (x, x)
ifn = 1
f (x, f(x, x))
ifn = 2
 in general,
f(x, f(x, f(x, ..., f(x, x))))
, wheref
is applied exactlyn
times.
Function compound f
need not behave well when applied to a negative integer.
Write algebraic laws for
compound
. Use as few base cases as possible.Implement
compound
.Use your
compound
function to define a Curried function for integer exponentiationval exp : int > int > int
so that, for example,
exp 3 2
evaluates to 9.
If you want to test compound
with any of the predefined infix operators, you will need to convert the infix name to “nonfix” in an expression like
compound (op +) 10 1
Don’t get confused by infix vs nonfix operators. Remember this:
 Fixity is a property of an identifier, not of a value.
 If
<$>
is an infix identifier, thenx <$> y
is syntactic sugar for<$>
applied to a pair containingx
andy
, which can also be written asop <$> (x, y)
.
Exceptions
Related Reading for problems L and M: In Learning Standard ML, read the section on Curried functions. Read the sections on Types (III) and Data (IV). Make sure you understand the difference between types and datatypes. Read the section on Exceptions, and make sure you know both how to raise
and how to handle
an exception.
L. Write a (Curried) function
val nth : int > 'a list > 'a
to return the nth element of a list. (Number elements from 0.) If nth
is given arguments on which it is not defined, raise a suitable exception. You may define one or more suitable exceptions or you may choose to use an appropriate one from the initial basis. (If you have doubts about what’s appropriate, play it safe and define an exception of your own.)
I expect you to implement nth
yourself and not simply call List.nth
.
M. Environments with exceptions.
In file
envdata.sml
, write definitions of a type'a env
and functionstype 'a env = (* you fill in this part *) exception NotFound of string val emptyEnv : 'a env = (* ... *) val bindVar : string * 'a * 'a env > 'a env = (* ... *) val lookup : string * 'a env > 'a = (* ... *)
such that you can use
'a env
for a type environment or a value environment. On an attempt to look up an identifier that doesn’t exist, raise the exceptionNotFound
. Don’t worry about efficiency.You can test your work interactively in
mosml
:Moscow ML version 2.103 (Tufts University, April 2012) Enter `quit();' to quit.  use "envdata.sml";  use "envdatatest.sml";
If all is well, you’ll get a bunch of messages about names and types, but no errors.
In file
envfun.sml
, do the same, except maketype 'a env = string > 'a
, and letfun lookup (name, rho) = rho name
As above, you can test using
mosml
andenvfuntest.sml
In file
envboth.sml
, write a functionval isBound : string * 'a env > bool
that works with both representations of environments. That is, write a single function that works regardless of whether environments are implemented as lists or as functions. You will need imperative features, like sequencing (the semicolon). Don’t use
if
.Test your code by running the following shell commands:
cat envdata.sml envboth.sml  mosml P full cat envfun.sml envboth.sml  mosml P full
Also in file
envboth.sml
, write a functionval extendEnv : string list * 'a list * 'a env > 'a env
that takes a list of variables and a list of values and adds the corresponding bindings to an environment. It should work with both representations. Do not use recursion. Hint: you can do it in two lines using the higherorder list functions defined above. You will have to copy the relevant functions into
envboth.sml
.Test your code by running the following shell commands:
cat envdata.sml envboth.sml  mosml P full cat envfun.sml envboth.sml  mosml P full
Algebraic data types
Related Reading for problems N and O: In Learning Standard ML, read the section on datatypes—Data IV. Make sure you understand how to pattern match on constructed values.
N. Search trees.
ML can easily represent binary trees containing arbitrary values in the nodes:
datatype 'a tree = NODE of 'a tree * 'a * 'a tree
 LEAF
To make a search tree, we need to compare values at nodes. The standard idiom for comparison is to define a function that returns a value of type order
. As discussed in Ullman, page 325, order
is predefined by
datatype order = LESS  EQUAL  GREATER (* do not include me in your code *)
Because order
is predefined, if you include it in your program, you will hide the predefined version (which is in the initial basis) and other things may break mysteriously. So don’t include it.
We can use the order
type to define a higherorder insertion function by, e.g.,
fun insert cmp =
let fun ins (x, LEAF) = NODE (LEAF, x, LEAF)
 ins (x, NODE (left, y, right)) =
(case cmp (x, y)
of LESS => NODE (ins (x, left), y, right)
 GREATER => NODE (left, y, ins (x, right))
 EQUAL => NODE (left, x, right))
in ins
end
This higherorder insertion function accepts a comparison function as argument, then returns an insertion function. (The parentheses around case
aren’t actually necessary here, but I’ve included them because if you leave them out when they are needed, you will be very confused by the resulting error messages.)
We can use this idea to implement polymorphic sets in which we store the comparison function in the set itself. For example,
datatype 'a set = SET of ('a * 'a > order) * 'a tree
fun nullset cmp = SET (cmp, LEAF)
Write a function
val addelt : 'a * 'a set > 'a set
that adds an element to a set.
Write a function
val treeFoldr : ('a * 'b > 'b) > 'b > 'a tree > 'b
that folds a function over every element of a tree, rightmost element first. Calling
treeFoldr (op ::) [] t
should return the elements oft
in order. Write a similar functionval setFold : ('a * 'b > 'b) > 'b > 'a set > 'b
The function
setFold
should visit every element of the set exactly once, in an unspecified order.
O. Sequences with constanttime append.
Consider the following informal definition of a sequence, by cases:
A sequence is either empty, or it is a singleton sequence containing one element
x
, or it is one sequence followed by another sequence.
Define an algebraic datatype
'a seq
that corresponds to this definition of sequences. As always, your definition should contain one value constructor for each alternative.Define a function
scons : 'a * 'a seq > 'a seq
that adds a single element to the front of a sequence, using constant time and space.Define a function
ssnoc : 'a * 'a seq > 'a seq
that adds a single element to the back of a sequence, using constant time and space.Note: The order of arguments is the opposite of the order in which the results go in the data structure. That is, in
ssnoc (x, xs)
,x
followsxs
. (The arguments are the way they are so thatssnoc
can be used withfoldl
andfoldr
.)Define a function
sappend : 'a seq * 'a seq > 'a seq
that appends two sequences, using constant time and space.Define a function
listOfSeq : 'a seq > 'a list
that converts a sequence into a list containing the same elements in the same order. Your function must allocate only as much space as is needed to hold the result.Without using explicit recursion, define a function
seqOfList : 'a list > 'a seq
that converts an ordinary list to a sequence containing the same elements in the same order.
An immutable, persistent alternative to linked lists
Related Reading for problem P In Learning Standard ML, read the section on datatypes—Data IV. Make sure you understand how to pattern match on constructed values.
P. For this problem I am asking you to define your own representation of a new abstraction: the list with finger. A list with finger is a nonempty sequence of values, together with a “finger” that points at one position in the sequence. The abstraction provides constanttime insertion and deletion at the finger.
This is a challenge problem. The other problems on the homework all involve old wine in new bottles. To solve this problem, you have to think of something new.
Define a representation for type
'a flist
. (Before you can define a representation, you will want to study the rest of the parts of this problem, plus the test cases.)Document your representation by saying, in a short comment, what sequence is meant by any value of type
'a flist
.Define function
val singletonOf : 'a > 'a flist
which returns a sequence containing a single value, whose finger points at that value.
Define function
val atFinger : 'a flist > 'a
which returns the value that the finger points at.
Define functions
val fingerLeft : 'a flist > 'a flist val fingerRight : 'a flist > 'a flist
Calling
fingerLeft xs
creates a new list that is likexs
, except the finger is moved one position to the left. If the finger belonging toxs
already points to the leftmost position, thenfingerLeft xs
should raise the same exception that the Basis Library raises for array access out of bounds. FunctionfingerRight
is similar. Both functions must run in constant time and space.Please think of these functions as “moving the finger”, but remember no mutation is involved. Instead of changing an existing list, each function creates a new list.
Define functions
val deleteLeft : 'a flist > 'a flist val deleteRight : 'a flist > 'a flist
Calling
deleteLeft xs
creates a new list that is likexs, except the value
x to the left of the finger has been removed. If the finger points to the leftmost position, thendeleteLeft
should raise the same exception that the Basis Library raises for array access out of bounds. FunctiondeleteRight
is similar. Both functions must run in constant time and space. As before, no mutation is involved.Define functions
val insertLeft : 'a * 'a flist > 'a flist val insertRight : 'a * 'a flist > 'a flist
Calling
insertLeft (x, xs)
creates a new list that is likexs, except the value
x is inserted to the left of the finger. FunctioninsertRight
is similar. Both functions must run in constant time and space. As before, no mutation is involved. (These functions are related to “cons”.)Define functions
val ffoldl : ('a * 'b > 'b) > 'b > 'a flist > 'b val ffoldr : ('a * 'b > 'b) > 'b > 'a flist > 'b
which do the same thing as
foldl
andfoldr
, but ignore the position of the finger.
Here is a simple test case, which should produce a list containing the numbers 1 through 5 in order. You can use ffoldr
to confirm.
val test = singletonOf 3
val test = insertLeft (1, test)
val test = insertLeft (2, test)
val test = insertRight (4, test)
val test = fingerRight test
val test = insertRight (5, test)
You’ll want to test the delete
functions as well.
Hints: The key is to come up with a good representation for “list with finger.” Once you have a good representation, the code is easy: over half the functions can be implemented in one line each, and no function requires more than two lines of code.
One problem you can do with a partner (15%)
The goal of this problem is to give you practice working with an algebraic data type that plays a central role in programming languages: expressions. In the coming month, you will write many functions that consume expressions; this problem will get you off to a good start. It will also give you a feel for the kinds of things compiler writers do.
Related Reading for Exercise 2: Ramsey, Section 5.10, which starts on page 368. Focus on the proof system for judgment y ∈ fv(e); it is provable exactly when freeIn e y
, where freeIn
is the most important function in Exercise 2. Also read function eval
in Section 5.4. You will modify the case for evaluating LAMBDA
.
2. In this problem we will follow up on a discussion I hope to have in class about how a compiler works. When it translates a lambda expression, a compiler doesn’t need to store an entire environment in a closure; it only needs to store the free variables of the lambda expression. This problem appears in Ramsey’s book as exercise 2 on page 375, and you’ll solve it in a prelude and four parts:
The prelude is to go to your copy of the book code and copy the file
bare/uschememl/mlscheme.sml
to your working directory. (This code contains all of the interpreter from Chapter 5.) Then make another copy and name itmlschemeimproved.sml
. You will editmlschemeimproved.sml
.The first part is to implement the freevariable predicate
val freeIn : exp > name > bool
.This predicate tells when a variable appears free in an expression. It implements the proof rules in section 5.10 of the book, which starts on page 368.
During this part I recommend that you compile early and often using
/usr/sup/bin/mosmlc c mlschemeimproved.sml
The second part is to write a function that takes a pair consistent of a
LAMBDA
body and an environment, and returns a better pair containing the sameLAMBDA
body paired with an environment that contains only the free variables of theLAMBDA
. (In the book, in exercise 1 starting on page 374, this environment is explained as the restriction of the environment to the free variables.) I recommend that you call this functionimprove
, and that you give it the typeval improve : (name list * exp) * 'a env > (name list * exp) * 'a env
The third part is to use
improve
in the evaluation case forLAMBDA
, which appears in the book on page 357b. You simply applyimprove
to the pair that is already there, so your improved interpreter looks like this:(* more alternatives for [[ev]] for \uscheme 357b *)  ev (LAMBDA (xs, e)) = ( errorIfDups ("formal parameter", xs, "lambda") ; CLOSURE (improve ((xs, e), rho)) )
The fourth and final part is to see if it makes a difference. Compile both versions of the μScheme interpreter using MLton, which is an optimizing, nativecode compiler for Standard ML.
mlton verbose 1 output mlscheme mlscheme.sml mlton verbose 1 output mlschemeimproved mlschemeimproved.sml
(If plain
mlton
doesn’t work, try/usr/sup/bin/mlton
.)
I have provided a script that you can use to measure the improvement. I also recommend that you compare the performance of the ML code with the performance of the C code in the course directory.
time runexponentialargmax 22 ./mlscheme
time runexponentialargmax 22 ./mlschemeimproved
time runexponentialargmax 22 /comp/105/bin/uscheme
Hints:
Focus on function
freeIn
. This is the only recursive function and the only function that requires case analysis on expressions. And it is the only function that requires you to understand the concept of free variables. You will be using all of these concepts on future assignments.Understanding free variables is hard, but once you understand, the coding is easy.
In Standard ML, the μScheme function
exists?
is calledList.exists
. You’ll have lots of opportunities to use it. If you don’t use it, you’re making extra work for yourself.In addition to
List.exists
, you may have a use formap
,foldr,
foldl, or
List.filter`.You might also have a use for these functions:
fun fst (x, y) = x fun snd (x, y) = y fun member (y, []) = false  member (y, z::zs) = y = z orelse member (y, zs)
The case for
LETSTAR
is gnarly, and writing it adds little to the experience. Here are two algebraic laws which may help:freeIn (LETX (LETSTAR, [], e)) y = freeIn e y freeIn (LETX (LETSTAR, b::bs, e)) y = freeIn (LETX (LET, [b], LETX (LETSTAR, bs, e))) y
It’s easier to write
freeIn
if you use nested functions. Mostly the variable y doesn’t change, so you needn’t pass it everywhere. You’ll see the same technique used in theeval
andev
functions in the chapter.If you can apply what you have learned on the
scheme
andhofs
assignments, you should be able to writeimprove
on one line, without using any explicit recursion.Let the compiler help you: compile early and often.
My implementation of freeIn
is 21 lines of ML.
Extra credit
There are two extracredit problems: FIVES and VARARGS.
FIVES
Consider the class of wellformed arithmetic computations using the numeral 5. These are expressions formed by taking the integer literal 5, the four arithmetic operators +, , *, and /, and properly placed parentheses. Such expressions correspond to binary trees in which the internal nodes are operators and every leaf is a 5. If you enumerate all such expressions, you can answer these questions:
What is the smallest positive integer than cannot be computed by an expression involving exactly five 5’s?
What is the largest prime number that can computed by an expression involving exactly five 5’s?
Exhibit an expression that evaluates to that prime number.
Write an ML function reachable
of type
('a * 'a > order) * ('a * 'a > 'a) list > 'a > int > 'a set
such that reachable (Int.compare, [op +, op , op *, op div]) 5 5
computes the set of all integers computable using the given operators and exactly five 5’s. (You don’t have to bother giving the answers to the questions above; the first two are easily gotten with setFold
.) My solution is under 20 lines of code. Such brevity is possible only because I rely heavily on the setFold
, nullset
, addelt
, and pairfoldr
functions defined earlier.
Hints:
 In order to be able to use
Int.compare
interactively, you will either have to runmosml P full
or else tell Moscow ML interactively toload "Int";
Begin your function definition this way:
fun reachable (cmp, operators) five n = (* produce set of expressions reachable with exactly n fives *)
 Use dynamic programming.
 Create a list of length k1 in which element i is a set containing all the integers that can be computed using exactly i elements. Now compute the kth element of the list by combining 1 with k1, 2 with k2, etcetera.
 Try doing the above by passing a list and its reverse, then use
pairfoldr
with a suitable function.  The initial list contains a set with exactly one element (in the example above, 5).
Make sure your solution has the completely general type given above, so you could use it with different operations and with different representations of numbers.
VARARGS
Extend μScheme to support procedures with a variable number of arguments. This is Exercise 8 on page 377.
Make sure your solutions have the right types
On this assignment, it is a very common mistake to define functions of the wrong type. You can protect yourself a little bit by loading declarations like the following after loading your solution:
(* first declaration for sanity check *)
val sameLength : 'a list * 'b list > bool = sameLength
val compound : ('a * 'a > 'a) > int > 'a > 'a = compound
val exp : int > int > int = exp
val fib : int > int = fib
val firstVowel : char list > bool = firstVowel
val null : 'a list > bool = null
val rev : 'a list > 'a list = rev
val minlist : int list > int = minlist
val foldl : ('a * 'b > 'b) > 'b > 'a list > 'b = foldl
val foldr : ('a * 'b > 'b) > 'b > 'a list > 'b = foldr
val zip : 'a list * 'b list > ('a * 'b) list = zip
val pairfoldr : ('a * 'b * 'c > 'c) > 'c > 'a list * 'b list > 'c = pairfoldr
val unzip : ('a * 'b) list > 'a list * 'b list = unzip
val concat : 'a list list > 'a list = concat
val nth : int > 'a list > 'a = nth
val addelt : 'a * 'a set > 'a set = addelt
val treeFoldr : ('a * 'b > 'b) > 'b > 'a tree > 'b = treeFoldr
val setFold : ('a * 'b > 'b) > 'b > 'a set > 'b = setFold
val scons : 'a * 'a seq > 'a seq = scons
val ssnoc : 'a * 'a seq > 'a seq = ssnoc
val sappend : 'a seq * 'a seq > 'a seq = sappend
val listOfSeq : 'a seq > 'a list = listOfSeq
val seqOfList : 'a list > 'a seq = seqOfList
val singletonOf : 'a > 'a flist = singletonOf
val atFinger : 'a flist > 'a = atFinger
val fingerLeft : 'a flist > 'a flist = fingerLeft
val fingerRight : 'a flist > 'a flist = fingerRight
val deleteLeft : 'a flist > 'a flist = deleteLeft
val deleteRight : 'a flist > 'a flist = deleteRight
val insertLeft : 'a * 'a flist > 'a flist = insertLeft
val insertRight : 'a * 'a flist > 'a flist = insertRight
val ffoldl : ('a * 'b > 'b) > 'b > 'a flist > 'b = ffoldl
val ffoldr : ('a * 'b > 'b) > 'b > 'a flist > 'b = ffoldr
(* last declaration for sanity check *)
I don’t promise to have all the functions and their types here—for example, this list includes only functions from warmup.sml
, not functions in envdata.sml
, envfun.sml
, or envboth.sml
. Making sure that every function has the right type is your job, not mine.
Avoid other common mistakes
It’s a common mistake to use any of the functions length
, hd
, and tl
. Instant No Credit.
If you redefine a type that is already in the initial basis, code will fail in baffling ways. (If you find yourself baffled, exit the interpreter and restart it.) If you redefine a function at the toplevel loop, this is fine, unless that function captures one of your own functions in its closure.
Example:
fun f x = ... stuff that is broken ...
fun g (y, z) = ... stuff that uses 'f' ...
fun f x = ... new, correct version of 'f' ...
You now have a situation where g is broken, and the resulting error is very hard to detect. Stay out of this situation; instead, load fresh definitions from a file using the use
function.
Never put a semicolon after a definition. I don’t care if Jeff Ullman does it, but don’t you do it—it’s wrong! You should have a semicolon only if you are deliberately using imperative features.
It’s a common mistake to become very confused by not knowing where you need to use op
. Ullman covers op
in Section 5.4.4, page 165.
It’s a common mistake to include redundant parentheses in your code. To avoid this mistake, use the checklist in the section Expressions VIII (Parentheses) in Learning Standard ML.
What to submit and how to submit it
There is no README file for this assignment.
Submitting your individual work
For your individual work, please submit the files cqs.ml.txt
, warmup.sml
, envdata.sml
, envfun.sml
, and envboth.sml
. If you have done either of the extracredit problems, submit them as varargs.sml
or fives.sml
.
In comments at the top of your warmup.sml
file, please include your name, the names of any collaborators, and the number of hours you spent on the assignment.
As soon as you have a warmup.sml
file, create empty files envdata.sml
, envfun.sml
, and envboth.sml
, and run submit105mlsolo
to submit a preliminary version of your work. As you edit your files, keep submitting; we grade only the last submission.
Submitting your improved μScheme interpreter
For your your improved μScheme interpreter, which you may have done with a partner, please submit the file mlschemeimproved.sml
, using the script submit105mlpair
.
How your work will be evaluated
The criteria are mostly the same as for the scheme
and hofs
assignments, but because the language is different, we’ll be looking for indentation and layout as described in the Style Guide for Standard ML Programmers.