SML basics Aslan Askarov aslan@cs.au.dk Revised from slides by E. - - PowerPoint PPT Presentation

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SML basics

Aslan Askarov aslan@cs.au.dk

Revised from slides by E. Ernst

ML

• Functional programming language
• Development initiated by Robin Milner at Edinburgh

in 1970s

• First implementation in 1974 on PDP-10
• Evolved over a period of 20+ years
• Fusion of many ideas from many people
• Initial application: DSL for manipulating logical proofs
• ML = Meta Language
• Tremendous influence on the design of modern

programming languages

• Type inference, polymorphism, safety, …

Robin Milner

“In 1956 […] I regarded programming as really rather

• inelegant. You’d write one instruction after the other, and

it was all rather arbitrary. It seemed to me that programming was not a very beautiful thing. So I resolved that I would never go near a computer in my life!” – R. Milner (interview by M. Bergen, 2003) ACM Turing Award, 1991

Honorary Doctorate, Aarhus University, 1998

“For three distinct and complete achievements: 1) LCF , the mechanization of Scott's Logic of Computable Functions, probably the first theoretically based yet practical tool for machine assisted proof construction; 2) ML, the first language to include polymorphic type inference together with a type-safe exception-handling mechanism; 3) CCS, a general theory of concurrency. In addition, he formulated and strongly advanced full abstraction, the study of the relationship between

• perational and denotational semantics.”

Standard ML

• SML is a dialect of ML
• Functional language
• Functions are first class (like in Scheme)
• The core syntactic category is expressions (not statements)
• Assignments are possible, but rare
• SML is statically and strongly typed
• No NullPointerExceptions, no ClassCastExceptions
• no casts, but many usages of pattern matching
• Everything has a type
• The interpreter (and the compiler) infers many types

automatically

• Variables are often declared w/o an explicit type

Using SML

• SML comes with a read-eval-print loop (like Python

and Scheme)

• All interactions must end with semicolon

> sml Standard ML of New Jersey v110.76 [built: Fri Aug 1 00:28:43 2014]

• print "hello, world!\n";

hello, world! val it = () : unit

Basic types (1/4)

• SML comes with a number of base types
• int, char, bool, string, real, …
• Integers are 31-bit: ~1, 0, 1, 42, Int.maxInt, …

all have type int

• int/real types come with usual operations: 


+, -, *, div, /, mod, …

• Note that unary negation is tilde, not minus
• Check out the associated libraries:

• pen Int; ..Char, Bool, String, Real

Basic types (2/4)

SML comes with

• Booleans: true and false both have type bool
• Boolean negation is not, conjunction is andalso,

and disjunction is orelse

• Usual comparison operations also produces bools:

=, <>, <, <=, >, >=, …

• Characters: #”a”, #”X”, #”\n”, #”\\”, #”\012”,

all have type char

• Conversions are done with chr: int -> char and
• rd : char -> int

Basic types (3/4)

SML comes with

• Strings: ””, ”hello, world!” both have type

string

• String concatenation is ^: ”d” ^ ”Ovs”
• One can inspect and manipulate strings:

String.size, String.substring, String.translate

• And convert to and from strings:

Int.fromString, Int.toString, Bool.fromString, Bool.toString, …

Basic types (4/4)

• () has type unit
• Notice how print returned unit
• unit serves the purpose of void in C and Java
• and doubles as the “empty argument (list)”:


SMLofNJ.SysInfo.getOSName()

• Technically it is not the “empty type” because one

value has the type unit, namely ()

• (* This is a (* nested *) comment in SML *)

Conditionals

Conditionals in SML are expressions and hence have a types y + (if x < 0 then ~x else x) Both then and else branch are required, and must have the same type if x > 0 then ”hello” else () Error: types of if branches do not agree [tycon mismatch] then branch: string else branch: unit in expression: …