Defining and Modifying Identifiers¶
As shown in the examples above, global identifiers are introduced using ‘set’, while local identifiers are introduced with ‘let’. Local identifiers can be used only in the expression after the following ‘in’, which may however be an arbitrarily large composite expression or clause. Note that in both cases the initial keyword is necessary; without it the ‘=’ would be interpreted as equality test (probably leading to an error due to an undefined left operand).
For modifying an existing variable, global or local, the assignment operator ‘:=’ is provided. It is mainly intended for programming use rather than for expression evaluation. For instance, a function returning a list of numbers 1,...,n may be written as follows (while-loops are explained later):
atlas> set initial (int n) = let i=0 in while i<n do i:=i+1 od Defined initial: (int->[int])
Here the part ‘let i=0 in’ serves to introduce ‘i’ as a variable, whose value is then repeatedly modified by the assignment ‘i:=i+1’. (The values from the assignments in the successive iterations of the loop body are then gathered into a row-of-integer value; a property of axis that is uncommon in other languages.) Any identifier can be used as a variable, including one introduced as a function argument; for instance for a decreasing list n-1,...,1,0 one could write:
atlas> set initial_downwards (int n) = while (n:=n-1)>=0 do n od Defined initial_downwards: (int->[int])
For identifiers holding an array, vector or matrix, one can also assign to individual entries, as in ‘v[i]:=x’ or ‘m[i,j]=i+j’; for matrices one can also assign to a whole column, provided the vector that replaces it has the correct size; for instance m[i]:=m[i]+m[j]*2 adds column j of m twice to its column i.
It is important to note that (unlike many programming languages) axis does not distinguish between values and objects. In other words two different names can never have any intimate relation (sharing an object) to each other, so assigning to one name will have no effect on the value associated to the other name. Therefore evaluating ‘let vi=v[i] in vi:=3’ does not have the same effect as evaluating v[i]:=3, as this would suppose that assigning to vi changes the (vector) value associated to v. Instead the value of the local name vi is assigned to just before that name disappears, so there is no side effect at all and the expression (it just returns 3), which then is rather pointless. By the same token calling functions is conceptually strictly by value (even if the implementation often avoids duplicating values); a function is allowed to assign to the names of its arguments, but this has no effect for the caller. For instance the assignment to its argument in the above function ‘initial_downwards’ does not have any affect for callers of this function, who do not know about the identifier ‘n’ in ‘initial_downwards’, or of its use. To pass information back from function to its caller, use the return value(s).
As a consequence of this principle, an assignment like v[i]:=x is really an assignment to the name v; this value is replaced by a value differing only at index i from the value it previously held.
(There is one way a function f can alter values of names of its caller g, but this requires that f be defined as a local function inside the body of g. Now f can, apart from its own parameters and local variables, can also “see” the names of g that are accessible at the point where f is defined, and just like any subexpression of the body of g, the body of f can assign to those names.)
In some cases it is desirable to introduce a name, but to forbid any form of assignment to that name; for instance if the value of a global variable is used inside a function, it can be important to ensure that the value obtained is that at the time the function was defined. Such constant names can be introduced by prefixing the name by ‘!’, as in ‘set !s = Split:(0,1)’ (this applies to local names as well as to global ones, and to function parameters, and also to components of a pattern matching a tuple value). Names introduced locally in loops (as described below) are automatically considered constant in this sense, since assigning to them has little effect and is usually an error.
As a convenience one can write ‘x +:= y’ instead of ‘x := x + y’, and similarly for any operator in the place of ‘+’, so in the above examples one could say i+:=1 and n-:=1. This is a transformation performed in the parser, so ‘+:=’ is not really an operator (and is composed of separate ‘+’ and ‘:=’). It should be noted that this requires a simple variable as left operand: the parser will reject for instance v[i]+:=1; this should be written v[i]:=v[i]+1.
The rules for assignment are quite different from the introduction of a variable (by ‘set’ or ‘let’): the variable must have been introduced beforehand, and the value assigned must have the same type as the previously held value. An assignment need not be used exclusively for the change to its variable it causes; it is also a subexpression in its own right whose value is the value assigned (both for ordinary and for component assignments); we already used this fact in the examples above.