Standard ML- Part I Compiler Baojian Hua

Standard ML is a Functional Language Small, orthogonal core based on the lambda-calculus. Control is based on (recursive) functions. Instead of for-loops, while-loops, do-loops, iterators, etc. rather, we can define these as library (functions). Makes it easy to define semantics. Supports higher-order, lexically-scoped, first-class functions. a.k.a. closures or lambdas. first-class: can be passed to other procedures as arguments, returned from procedures as results, placed in data structures etc. lexically-scoped: meaning of variables determined statically. higher-order: compositional, programs that build programs. These aspects are in common with other functional languages such as Common Lisp, Scheme, Haskell, Ocaml, F#, and Erlang.

Standard ML is mostly Pure As opposed to mostly imperative languages. C/C++, C#, Java, Python, Basic, Fortran, … Emphasis on values, not variables or state: “ value-oriented programming ” build new values out of old, instead of destructively updating the old. leads to a simple analytic, computational model increasingly important in age of concurrency Still provides for mutable data structures and I/O. for efficiency of some algorithms but less frequently than you might first suspect.

Standard ML is Strongly, Statically Typed Statically typed: compiler catches many silly errors before you can run the code. e.g., calling a function with the wrong types of arguments Strong typed: compiler enforces type abstraction. cannot cast an integer to a record, function, string, etc. so we can utilize types as capabilities.

A bit of history Alonzo Church: lambda calculus 1930’s John McCarthy: LISP 1958 Guy Steele & Gerry Sussman: Scheme late 1970’s Robin Milner, Mads Tofte, & Robert Harper Standard ML 1980’s Xavier Leroy: Ocaml 1990’s Don Syme: F# 2000’s

Standard ML Standard ML is a domain-specific language for building compilers, interpreters, theorem provers, etc. Features First-class functions Memory management like Java Exception handling Module system supporting large projects Advanced type system for error detection

Introduction to SML You will be responsible for learning SML on your own. Today I will cover some basics Readings: Robert Harper’s Online book “Programming in Standard ML” is a good place to start Also see course webpage for pointers and info about how to get, install and run the software

Installation and Run We ’ d use SML/NJ and MLton, see the webpage for detailed info ’ easy installation on Linux, Windows, … Editing Notepad, Eclipse, VS, Emacs, Vi, … Next, I ’ d introduce SML via examples

Preliminaries Start sml in Unix or Windows by typing sml at a prompt: % sml Standard ML of New Jersey, v […] - (* quit SML by pressing ctrl-D; ctrl-Z *some times... *) (* just as you see, comments can be (* nested *) *)

Preliminaries Read – Eval – Print – Loop ;

Preliminaries Read – Eval – Print – Loop ; > val it = 5 : int

Preliminaries Read – Eval – Print – Loop ; > val it = 5 : int - it + 7; > val it = 12 : int

Preliminaries Read – Eval – Print – Loop ; > val it = 5 : int - it + 7; > val it = 12 : int - it – 3; > val it = 9 : int true; stdIn: 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; uncaught exception Div [divide by zero] run-time error

Basic Values - (); > val it = () : unit => like “void” in C (sort of) => the uninteresting value/type - true; > val it = true : bool - false; > val it = false : bool - if it then 3+2 else 7; “else” clause is always necessary > val it = 7 : int - false andalso true; > val it = false : booland also, or else short-circuit eval

Basic Values Integers ; > val it = 5 : int (if not true then 5 else 7); > val it = 10 : int No division between expressions and statements Strings - “Baojian” ^ “ “ ^ “Hua”; > val it = “Baojian Hua” : string - print “foo\n”; foo > val it = () : unit

Declarations (bindings) Just like in any other language, SML allows value and type declarations with key difference that values in SML don ’ t vary so it ’ s called value binding (rather than value assignment) Basic forms: val id: type = exp type id = typeExp

First-class Functions It ’ s simple, it ’ s “ fun ” fun (id: type): type = exp The power of SML functions comes from the fact that they are first-class stored in data structures passed as arguments, returned as values nested! (closure)

Algebraic Data Types A simple yet powerful feature allow user-defined data type like unions or inheritance in other languages with the key highlights of type-safety may be non-recursive or recursive natural to represent compiler intermediate languages

Pattern Matching Pattern matching for handling case analysis on algebraic Exhaustiveness Redundancy

List The ubiquitous data structure in functional programming See the accompany code for details

Reference Normally, variables in SML don ’ t vary, if you ’ d like to modify a variable, use reference think “ malloc ” or “ new ” in other languages again with type-safety assurance should be used rarely and with care

More on 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 declarations with 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 “ ~/src ” ; - CM.make “ sources.cm ” ; looks for “ sources.cm ”, analyzes dependencies [compiling … ] compiles files in group [wrote … ] saves binaries in.cm/ sources.cm c.smlb.smla.sml Group is a.sml b.sml c.sml

Core Language All SML expressions produce values that have a particular type SML doesn’t have “statements” SML can do type inference (and give you hard-to-decrypt error messages) SML data types are super-cool a new type name new constructors new patterns In summary, SML has a small, elegant yet power core language Next time, module system

Summary Learning to program in SML can be tricky at first But once you get used to it, you will never want to go back to imperative languages Check out the reference materials listed on the course homepage, do reading and practicing