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Introduction to ML. You will be responsible for learning ML on your own. Today I will cover some basics Read Robert Harper’s notes on “an introduction to ML” See course webpage for pointers and info about how to get the software. Intro to ML. Highlights Functional Language
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Introduction to ML • You will be responsible for learning ML on your own. • Today I will cover some basics • Read Robert Harper’s notes on “an introduction to ML” • See course webpage for pointers and info about how to get the software
Intro to ML • Highlights • Functional Language • Functions are pervasive: • First-class values • Storable, arguments, results, nested • 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)
Interactive Language • Type in expressions • Evaluate and print type and result • Statically Scoped Language • Compile-time resolution of values • Call-by-value language • f(e) • High-level programming features • Data types • Pattern matching • Exceptions • Mutable data discouraged
Preliminaries • Read – Eval – Print – Loop - 3 + 2; > 5: int - it + 7; > 12 : int - it – 3; > 9 : int - 4 + true; Type clash in : 3 + true Looking for a : int Type I have found a : bool Error - 3 div 0 Failure : Div - run-time error
Basic Values - (); > () : unit => like “void” in c (sort of) => the uninteresting value/type - true; > : bool - False; > : bool - if it then 3+2 else 7; always necessary > 7 : int - false and also loop_Forever; > False : bool and also, or else short-circuit eval
Basic Values Integers -3 + 2j > 5:int - 3 + (if not true then 5 else 7); > 10 : int No division between expressions and statements Strings - “Dave” ^ “ “ ^ “Walker”; > “Dave Walker”:string - size “foo”; > 3 : int Reals - 3.14; > 3.14 : real
Structured Values Tuples - (true, 17, “Stuff”); > (true, 17, “Stuff”) : bool * int * string - (if 3 < 2 then “x” else “y”, false); > (“y”, false) : string * bool Records - {name = “Dave”, ssn = 332177} > _____________ : {name = string, ssn : int} - #name it ; > “Dave” : string - #1 (true, False); - true : bool
Parametric Polymorphism • Functions like compose work on objects of many different types • In ML, we write compose: (‘a -> ‘b) -> (‘c -> ‘a) -> type variables are written with ‘(‘c -> ‘b) • compose not : • compose not not : • compose (op + 1) not : • compose (fn x => x) :
What is the type of compose? • Fun compose = Fn f => (fn g => Fn x => f (g x)) T3 Compose : T2 -> (T2 -> (T5 -> T6) : T2 -> (( T7 -> T8) -> (T5 -> T6)) : (T9 -> TA) -> (T7 -> T9) -> (T7 -> TA) : (‘a -> ‘b) -> (‘c -> ‘a) -> (‘a -> ‘c) Compose (fn x => x + 1) (fn y => y * 3) 7;
Declarations - val x = 4 * 5; Declare new name x and bind it to value > val x = 20 : int - val y = x + 1; > val y = 21 : int - val x = x + y; reuse name > val x = 41 : int - x; > 41 : int static scoping - y; local declaration > 21 : int scope - (let val x = 10 in (x + x, y + x) and, x); > ((20, 31), 41) : (int * int) * int
Functions (the good stuff) - not; functions are values > Not = fn : bool -> bool we can’t write out a function so ML just says this - not true; function application is a space > False : bool - fun twice x = 2 * x > Val twice = fn : int -> int - fun fact x = if x = 0 then 1 else (fact (x-1)) * x - fact (twice (twice 3));
functions - fn s => x ^ “!”; an anonymous function like writing > Fn : string -> string -3 - val exclaim = rather than - val x = 3; fn : => s ^ “!” > … - val compose = fn f => fn g => fn x => f(g x); > … - ((compose twice) twice) 3; > 12 - four = compose twice twice > … - four 7; > …