Introduction to ML – Part 1

Frances Spalding

Assignment 1 http://www.cs.princeton.edu/cours

es/archive/fall05/cos441/assignments/a1.htm

Due next Monday (Oct 3rd)

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Standard ML Standard ML is a domain-specific

language for building compilers Support for

Complex data structures (abstract syntax, compiler intermediate forms)

Memory management like Java Large projects with many modules Advanced type system for error

detection

Introduction to ML You will be responsible for learning ML

on your own. Today I will cover some basics Aquinas will do a second class on ML on

Thursday Resources:

Jeffrey Ullman “Elements of ML Programming” Robert Harper’s “an introduction to ML” See course webpage for pointers and info about

how to get the software

Intro to ML Highlights

Data Structures for compilers Data type definitions Pattern matching

Strongly-typed language Every expression has a type Certain errors cannot occur Polymorphic types provide flexibility

Flexible Module System Abstract Types Higher-order modules (functors)

Intro to ML Interactive Language

Type in expressions Evaluate and print type and result Compiler as well

High-level programming features Data types Pattern matching Exceptions Mutable data discouraged

Preliminaries Read – Eval – Print – Loop

- 3 + 2;

Preliminaries Read – Eval – Print – Loop

- 3 + 2;> 5: int

Preliminaries Read – Eval – Print – Loop

- 3 + 2;> 5: int- it + 7;> 12 : int

Preliminaries Read – Eval – Print – Loop

- 3 + 2;> 5: int- it + 7;> 12 : int- it – 3;> 9 : int- 4 + true;

stdIn:17.1-17.9 Error: operator and operand don't agree [literal]

operator domain: int * int operand: int * bool in expression: 4 + true

Preliminaries Read – Eval – Print – Loop

- 3 div 0;Failure : Div - run-time error

Basic Values- ();> () : unit => like “void” in C (sort of)

=> the uninteresting value/type

- true;> true : bool- false;> false : bool- if it then 3+2 else 7; “else” clause is always necessary> 7 : int- false andalso loop_Forever;> false : bool and also, or else short-circuit eval

Basic ValuesIntegers- 3 + 2> 5 : int- 3 + (if not true then 5 else 7);> 10 : int No division between expressions

and statementsStrings- “Dave” ^ “ “ ^ “Walker”;> “Dave Walker” : string- print “foo\n”;foo> 3 : int

Reals- 3.14;> 3.14 : real

Using SML/NJ Interactive mode is a good way to

start learning and to debug programs, but…

Type in a series of declarations into a “.sml” file

- use “foo.sml”[opening foo.sml]…

list of declarationswith their types

Larger Projects SML has its own built in interactive

“make” Pros:

It automatically does the dependency analysis for you

No crazy makefile syntax to learn Cons:

May be more difficult to interact with other languages or tools

Compilation Manager

% sml

- OS.FileSys.chDir “~/courses/510/a2”;

- CM.make(); looks for “sources.cm”, analyzes dependencies

[compiling…] compiles files in group

[wrote…] saves binaries in ./CM/

- CM.make’ “myproj/”(); specify directory

sources.cmc.smlb.smla.sigGroup is

a.sigb.smlc.sml

What is next? ML has a rich set of structured values

Tuples: (17, true, “stuff”) Records: {name = “Dave”, ssn = 332177} Lists: 3::4::5::nil or [3,4]@[5] Datatypes Functions And more!

Rather than list all the details, we will write a couple of programs

An interpreter Interpreters are usually

implemented as a series of transformers:

stream ofcharacters

abstractsyntax

lexing/parsing

evaluate

abstractvalue

print

stream ofcharacters

A little language (LL) An arithmetic expression e is

a boolean value an if statement (if e1 then e2 else e3) an integer an add operation a test for zero (isZero e)

LL abstract syntax in ML

datatype term = Bool of bool| If of term * term * term| Num of int| Add of term * term| IsZero of term

-- constructors are capitalized

-- constructors can take a single argument of a particular type

type of a tupleanother eg: string * char

vertical barseparates alternatives

LL abstract syntax in ML

Add (Num 2, Num 3)

represents the expression “2 + 3”

Add

Num Num

2 3

LL abstract syntax in ML

If (Bool true, Num 0, Add (Num 2, Num 3))

represents

“if true then 0 else 2 + 3”

Add

Num Num

2 3

true

Bool Num

0

If

Function declarations

fun isValue t = case t of Num n => true | Bool b => true | _ => false

function name function parameter

default pattern matches anything

What is the type of the parameter t? Of the function?

fun isValue t = case t of Num n => true | Bool b => true | _ => false

function name function parameter

default pattern matches anything

What is the type of the parameter t? Of the function?

fun isValue (t:term) : bool = case t of Num n => true | Bool b => true | _ => false

val isValue : term -> bool

ML does type inference => you need notannotate functions yourself (but it can be helpful)

A type error

fun isValue t = case t of Num _ => true | _ => false

ex.sml:22.3-24.15 Error: types of rules don't agree [literal] earlier rule(s): term -> int this rule: term -> bool in rule: _ => false

A type error

Actually, ML will give you several errors in a row:

ex.sml:22.3-25.15 Error: types of rules don't agree [literal] earlier rule(s): term -> int this rule: term -> bool in rule: Successor t2 => trueex.sml:22.3-25.15 Error: types of rules don't agree [literal] earlier rule(s): term -> int this rule: term -> bool in rule: _ => false

A very subtle error

fun isValue t = case t of num => true | _ => false

The code above type checks. But whenwe test it refined the function always returns “true.”What has gone wrong?

A very subtle error

fun isValue t = case t of Num 0 => 1 | Add(Num t1,Num t2) => t1 + t2 | _ => 0

The code above type checks. But whenwe test it refined the function always returns “true.”What has gone wrong?-- num is not capitalized (and has no argument)-- ML treats it like a variable pattern (matches anything!)

Exceptions

exception Error of string

fun debug s : unit = raise (Error s)

Exceptions

exception Error of string

fun debug s : unit = raise (Error s)

- debug "hello";

uncaught exception Error raised at: ex.sml:15.28-15.35

in SML interpreter:

Evaluator

fun isValue t = ...

exception NoRule

fun eval t = case t of Bool _ | Num _ => t | ...

Evaluator

...

fun eval t = case t of Bool _ | Num _ => t | If(t1,t2,t3) => let val v = eval t1 in case v of Bool b => if b then (eval t2) else (eval t3) | _ => raise NoRule end

let statementfor rememberingtemporaryresults

Evaluatorexception NoRule

fun eval1 t = case t of Bool _ | Num _ => ... | ... | Add (t1,t2) => case (eval v1, eval v2) of (Num n1, Num n2) => Num (n1 + n2) | (_,_) => raise NoRule

Finishing the Evaluatorfun eval1 t = case t of ... | ... | Add (t1,t2) => ... | IsZero t => ...

be sure yourcase isexhaustive

Finishing the Evaluatorfun eval1 t = case t of ... | ... | Add (t1,t2) => ... What if we

forgot a case?

Finishing the Evaluator

ex.sml:25.2-35.12 Warning: match nonexhaustive (Bool _ | Zero) => ... If (t1,t2,t3) => ... Add (t1,t2) => ...

fun eval1 t = case t of ... | ... | Add (t1,t2) => ... What if we

forgot a case?