Useful Information for SML Summary

General machines on which SML is available:

1. schubert
2. beethoven
3. mozart
4. kern
5. walton

Using the interpreter:

• starting it: execute "sml" at unix prompt; (really /usr/local/bin/sml)

• exiting it: control-D

The intepreter prompt is a dash. At this prompt you can:

• evaluate an expression;
  EX: (note: the dash is the interpreter prompt)
  - 2 + 3;
  val it = 5 : int <== interpreter response
  

• define a function or a value:
  EX:
  - fun inc x = x + 1;
  val inc = fn : int -> int
  - inc 42;
  val it = 43 : int
  - val xyz = inc (inc 55);
  val xyz = 57 : int
  - inc (2 * xyz);
  val it = 115 : int

• load a file of "val" and "fun" definitions:
  EX:
  - use "testfile.sml";
  [opening testfile.sml]
  val inc = fn : int -> int
  val it = () : unit
  

• comments
  text between (* and *) is a comment, thus ignored;

Showing large data structures:

• System.Control.Print.printDepth := 25;
• System.Control.Print.printLength := 500;

Common problems you might have:

• syntax errors
  EX:
  - fun inc x) = x + 1;
  std_in:3.10 Error: syntax error found at RPAREN

• type errors
  EX:
  - fun f (x::xs) = x mod xs;
  std_in:0.0-0.0 Error: operator and operand don't
  agree (tycon mismatch)
  operator domain: int * int
  operand: int * int list
  in expression:
  mod (x,xs)
  Here we tried to do a "mod" operation with a list (xs). The type checker is telling us that the mod operator takes two integers (int * int) but that we applied it to an int and an int list (int * int list).

• overloading ambiguity
  EX:
  - fun square x = x * x;
  std_in:0.0 Error: overloaded variable cannot be resolved: *
  Here the system cannot decide whether we intend this function to be multiplying two integers or two reals, since the "*" operator is overloaded. We can fix this by telling it the type of its return value:
  - fun square x : int = x * x;
  val square = fn : int -> int
  Now it understands that we intend integer multiplication.

Basic types and operations:

type converter functions:
real : int -> real
floor : real -> int

Structured types (built in)

alternate form of list is
[a,b,c] <==> a::b::c::nil
[] <==> nil

User-defined types - EX:

• datatype shape = circle of real | rectangle of real * real;
  This has defined a type called "shape". Its constructor functions are "circle" and "rectangle". The circle function takes a real as its argument; the rectangle function takes a pair of reals as its argument.

• fun area (circle radius) = 3.14159 * radius * radius
  | area (rectangle (height, width)) = height * width;
  This has defined the function that calculates the area of a shape.

  - val block1 = rectangle (2.0, 5.0);
  val block1 = rectangle (2.0,5.0) : shape
  - val block2 = rectangle (3.5, 1.0);
  val block2 = rectangle (3.5,1.0) : shape
  - val disk1 = circle 4.7;
  val disk1 = circle 4.7 : shape
  - val disk2 = circle 8.0;
  val disk2 = circle 8.0 : shape
  - area block1 + area block2 + area disk1 + area disk2;
  val it = 283.9594831 : real
  We created two rectangles and two disks. Then we added up their areas.

  - val objects = [block1, block2, disk1, disk2];
  val objects = [rectangle (2.0,5.0),rectangle (3.5,1.0),
  circle 4.7,circle 8.0] : shape list
  - map area objects;
  val it = [10.0,3.5,69.3977231,201.06176] : real list
  We made a list of our four objects, and mapped the area function on the list, giving us a list of their areas.

  - val disks = map circle [2.0, 6.5, 7.1, 3.3, 2.5];
  val disks = [circle 2.0,circle 6.5,circle 7.1,circle 3.3,
  circle 2.5] : shape list
  We took a list of reals and mapped the circle function on it, giving us a list of circles.