Gedanken(ID:508/ged002)

Appendix. A Brief Comparison of Gedanken with Algol-60

The basic means of introducing new identifiers is the is-declaration which is used  in three ways here:

1.	Count is Ref(0)	is like	integer Count; Count := 0
2.	Locate is X[•••]	is like	integer procedure Locate(X);           begin • • • end
3.	I is Locate(X)	is like	integerI; I : = Locate(X)

References (addresses) are dealt with more explicitly. Refis a function which returns a new reference, containing its argument. If r is a reference, Val(r) is its contents, and r := e changes its contents to e. A set of coercion rules (not given here)  serve to insert applications of VAL so that, if Count is a reference,

Vectors and n-tuples are subsumed under the more general notion of functions. The expressions

(2,4,6)

and

(X if I < X< 3 then 2*X

else go to Error)

and

Vector(1,3,X2*X)

all denote the same thing: a function F such that F(1) = 2, F(2) = 4, F(3) = 6.

If Createseal()> returns a pair, the declaration

Seal, Unseal isCreateseal( )

serves to decompose it and bind the components to the two identifiers.

The general form for a (large) GEDANKEN expression is a sequence of (0 or more) is-declarations followed by (1 or more) expressions (X := y is an expression).

[d₁; • • • ;d_n;e₁;• • • e_m]

The declarations and expressions are evaluated from left to right and the value of the whole expression is e_m . The (textual) scope of the definitions is limited by the brackets.

To make the programs more readable we write key words of gedanken in boldface and capitalize the first letter of identifiers. Also variously-shaped brackets are used. Comments are written to the right of double slashes (//).