# Tuple Displays and Pattern Matching¶

If instead of brackets of a row display one uses parentheses to enclose a sequence of expressions, then a tuple rather than an array is formed (except in case there is exactly one expression in the sequence: then parentheses just imply grouping as used for instance to override operator precedence). Tuples differs from array in that there elements are not required to have the same type, and (therefore) cannot be selected by position. In fact the type of a tuples expression explicitly specifies the type of the component at each position, and since the type checker insists that all expressions have a type, it is not possible to work with a tuple not having a fixed number of components (unlike arrays, which can well be of varying length). Tuple displays are frequently used to pack the arguments of a function (officially every function takes a single argument, but very often it is of tuple type). So for instance ‘f(x,y)’ applies the function ‘f’ to a 2-tuple with components ‘x’ and ‘y’. But one is not obliged to write a tuple display as argument of a function expecting a tuple; for instance after ‘set z=(3,4)’ the variable ‘z’ holds a 2-tuple, and (the same instance of) ‘f’ may also be called as ‘f(z)’. One could even for instance multiply the components of ‘z’ by writing *z (giving 12 in the example); indeed the expression 3*4 is transformed by the parser into *(3,4), and (currently) this form is also used in error messages, for instance in:

atlas> GL(10/2)
Error in expression GL(/(10,2)) at <standard input>:...
Failed to match 'GL' with argument type rat
Type check failed


(The operator ‘/’ on integers produces a rational number; for integer division ignoring the remainder one should write 10\2 instead).

To decompose tuples, a simple form of pattern matching is provided: wherever an identifier can be introduced, one may also specify a tuple of identifiers, if the type of the value is appropriate, and the components will be bound to the corresponding identifiers. So whereas:

atlas> set x = E


introduces the identifier ‘x’ whose value (and type) are set to that of the expression ‘E’, one can alternatively say

atlas> set (x,y) = E


provided that E produces a pair (some value of some 2-tuple type), and the variable ‘x’ will be set to the first component of the pair, and ‘y’ to the second component. In either case, the types of the variables introduced will be reported, as in:

atlas> set (q,r) = 22 \% 7   { Euclidean division with remainder }
Identifiers q: int, r: int
atlas> q    { quotient }
Value: 3
atlas> r    { remainder }
Value: 1


The same pattern-matching is also allowed when introducing parameters of user-defined functions, or when introducing local variables. To introduce local variables one uses a syntax like that of ‘set’ used above, but with ‘let’ instead of ‘set’, and followed by ‘in’ and the “body” if the let-expression, the expression where the local variable is in scope. So to compute a value from the components of ‘z’, one could say:

atlas> let (x,y) = z in 3*y-x


If only the first component is needed, there is no need to give a name to the other one, so for the projection to the first component one can write:

atlas> let (x,)=z in x


Local variables get their value and type from the expression between ‘=’ and ‘in’, and are independent of any meaning that might previously be associated to the identifier; after evaluation of the body, the local identifiers introduced, and their values associated to them, are forgotten. (One can no longer refer to the local name outside the expression following ‘in’; in some cases references to them made from inside that expression may survive though.)

Using a local variable, the initial example could have been given in a form that does not permanently bind the identifier ‘ic’:

atlas> let ic = inner_class("B5.A3.T1",[[1/2,0,0],[0,1/2,1/2]],"scc")
L > in block_sizes(ic)
Value:
| 0,  0,   0,  0,   0,    0,    0,     1 |
| 0,  0,   0,  0,   0,    0,    0,    11 |
| 0,  0,   0,  0,   0,    0,    0,    10 |
| 0,  0,   0,  0,   0,    0,    0,    75 |
| 0,  0,   0,  0,   0,    0,    0,   110 |
| 0,  0,   0,  0,   0,    0,    3,    21 |
| 0,  0,   0,  0,   0,    0,    0,   305 |
| 0,  0,   0,  0,   0,    0,    0,   750 |
| 0,  0,   0,  0,   0,    0,   33,   231 |
| 0,  0,   0,  0,   0,    0,    0,   810 |
| 0,  0,   0,  0,   0,    0,    0,  3050 |
| 0,  0,   0,  0,   0,    0,  225,  1575 |
| 0,  0,   0,  1,  25,  130,    0,  1342 |
| 0,  0,   0,  0,   0,    0,    0,  8100 |
| 0,  0,   0,  0,   0,    0,  915,  6405 |
| 0,  0,   0, 10, 250, 1300,    0, 13420 |
| 0,  0,   0,  0,   0,    0, 2430, 17010 |
| 3, 75, 390, 21, 525, 2730, 4026, 28182 |


Note that axis recognized that the let-expression was incomplete after the first line, changing the prompt to ‘L >’ in the second line to indicate this.