SML97 Specifics

1
SML-97 Specifics

• SML/NJ 110

2
Getting Started

• Invoking interpreter (actually a compiler in
  interactive mode!)
• sml
• Loading ML program
• - use /eg.sml
• Accessing a structure element
• Int.max
• Opening a structure
• open Real

• Top-level environment contains a selected set of
  useful functions. A more complete set of standard
  functions is organized into structures (Int,
  Real, String, etc) that forms the standard basis.
• - Real.Math.sqrt(4.0)
• - List.take(2,1,2,3)

3
Expressions

• 1 21
• val it 20 int
• 1\t\na\t\n
• tab, newline,
• \\\ abc
• Slash, double quotes, concatenation
• - a
• character

• Z lt a
• ab lt acd
• 1 ltgt 2
• 1 lt 2 orelse 0/0
• if true andthen
• not (1gt2)
• then 3.0
• else 3.142
• if-then control structure is legal, but not
  expression.

4
Tip of the Type Errors

• 1 2.0
• coercion
• real(1) 2.0
• 1 floor(2.0)
• 1/2
• a bb
• if true
• then a else bb
• 12.0

• real is not an equality type.
• Portability reasons.
• (Cf. (int-gtint) is not an equality type for
  computability reasons.)
• In the absence of rounding errors, (rs) is
  equivalent to (rlts) andalso (sltr).

5
Identifiers

• Alphanumeric Identifiers
• a1_b2, B2_bomber, , etc
• ordinary variables
• abc, a, etc
• type variables
• Symbolic Identifiers
• ltgt, _at_, ?, , /, etc
• Separators
• ( , ) , , , , , , . , , ,
• ML is case-sensitive.

6
Variables

• Identifiers vs Variables vs Values
• Environment vs Store
• Adding new variable and binding
• val i 10
• val i 1,2,3
• (Cf., in Pascal and in C.)
• Static scoping (Closure)
• val a 2 fun ax(x) a x
• val a 5 ax 10
• ( ax 10 20 )

7
Patterns

• fun merge (L as xxs, M as yys)
• fun comb(_,0)
• fun f ((x,y)zxs)
• Illegal cases
• fun length (xs _at_ x)
• fun f (x1)
• fun g (0.0)
• Term Matching (cf. Unification)

8

• fun same (n,n) 0
• same _ 1
• Error duplicate variable in pattern
• ( requires equational reasoning )
• fun fp nil 0
• fp (xxs) 1
• val fp fn 'a list -gt int
• fun feq xs
• if (xs nil) then 0 else 1
• val feq fn ''a list -gt int

9
Match Expressions

• val rec reverse fn
• nil gt nil
• xxs gt reverse(xs) _at_ x
• rule pattern gt expression
• case x lt y of
• true gt x false gt y
• if x lt y then 1 else 1
• Error Type of rules do not agree.

10
Limitations on the use of polymorpic functions

• fun id x x ( id a -gt a )
• id 0 ( id int -gt int )
• id id 0 ( id (int -gt int) -gt (int -gt int) )
• val id fn x gt x ( id a -gt a )
• To ensure that a bound variable has the same
  meaning as its binding, the variable introduced
  by a val-declaration is allowed to be polymorphic
  only if the right-hand side is a value (and not
  a computation).

11

• val J id id (illegal)
• Motivation Contexts (such as (J 5, J a) )
  that replicate the binding may impose
  conflicting requirements on the type.
• (cf. Multiple evaluation of side-effect
  causing expressions.)
• (cf. Referential transparency violated if two
  occurrences of an expression is not equivalent to
  two occurrences of its value.)
• Value Restriction is too conservative.
• val k nil _at_ nil ( illegal )

12
Value Restriction on Polymorphic Declaration

• ILLEGAL
• id id
• id foldr
• LEGAL
• fun J x id id x
• id id a
• id Int.max
• let val v id id
• in v a end

13
Polymorphic type variable a

• Generalizable (universal quantification)
• for every type T, there is an instance of this
  object
• Non-generalizable (existential quantification)
• there exists a fixed but arbitrary type T for
  this object
• For compile-time guarantees, SML-97 requires
  that the expressions at the top-level be such
  that the generalizable interpretation of type
  variables is appropriate.

14
Non-expansive Expressions

• A constant or a variable is non-expansive.
• A function definition is non-expansive.
• A tuple of non-expansive expressions is
  non-expansive.
• A non-expansive expression may be preceded by a
  data/exception constructor.
• Expressions that are not of these forms are
  expansive and are not allowed to have type
  variables.
• Generates a type variable not generalizable
  error.

15
Non-generalizable type variable error

• All the following conditions must be met for the
  error to occur
• The expression is at the top-level.
• The type of the expression involves at least one
  type variable.
• The form of the expression does not meet the
  conditions for it to be non-expansive.

16
Examples

• id id ( error )
• (id, id)
• val it ltpoly-recordgt
• ('a -gt 'a)('b -gt 'b)
• id (idint-gt int)
• id, id
• val it fn,fn ('a -gt 'a) list
• ( Allowed by SML97 even though according
  to Ullmans book it is illegal? )

17
(contd)

• let val x (id id)
• in x(1) end
• ( Legal. (id id) is not top-level. )
• let val x (id id)
• in (x 1, x a) end
• ( Error operator and operand don't agree.)
• (id 1, id a)
• ( val it (1,"a") int string )
• let val x id
• in (x 1, x a) end