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_L_i_s_t_ _c_o_m_p_r_e_h_e_n_s_i_o_n_s
[exp | qualifiers]
List of all `exp' such that `qualifiers'. If there are two or more
qualifiers they are separated by semicolons. Each qualifier is either a
generator, of which the allowed forms are
pattern-list <- exp (first form)
pattern <- exp, exp .. (second form)
or else a filter, which is a boolean expression restricting the range of
the variables introduced by preceding generators. The variables
introduced on the left of each `<-' are local to the list comprehension.
Some examples
sqs = [ n*n | n<-[1..] ]
factors n = [ r | r<-[1..n div 2]; n mod r = 0 ]
knights_moves [i,j] = [ [i+a,j+b] | a,b<-[-2..2]; a^2+b^2=5 ]
Notice that a list of variables on the lhs of a `<-' is shorthand for
multiple generators, e.g. `i,j<-thing' expands to `i<-thing; j<-thing'.
The variables introduced by the generators come into scope from left to
right, so later generators can make use of variables introduced by
earlier ones. An example of this is shown by the following definition
of a function for generating all the permutations of a given list.
perms [] = [[]]
perms x = [ a:p | a<-x; p<-perms(x--[a]) ]
The second form of generator allows the construction of lists from
arbitrary recurrence relations, thus
x <- a, f x ..
causes x to assume in turn the values `a', `f a', `f(f a)', etc.
An example of its use is in the following definition of the fibonacci
series
fibs = [ a | (a,b) <- (1,1), (b,a+b) .. ]
Another example is given by the following expression which lists the
powers of two
[ n | n <- 1, 2*n .. ]
The order of enumeration of a list comprehension with multiple
generators is like that of nested for-loops, with the rightmost
generator as the innermost loop. For example the value of the
comprehension [ f x y | x<-[1..4]; y<-[1..4] ] is
[ f 1 1, f 1 2, f 1 3, f 1 4, f 2 1, f 2 2, f 2 3, f 2 4,
f 3 1, f 3 2, f 3 3, f 3 4, f 4 1, f 4 2, f 4 3, f 4 4 ]
As a consequence of this order of enumeration of multiple generators, if
any generator other than the first (leftmost) is infinite, some
combinations of values will never be reached in the enumeration. To
overcome this a second, _d_i_a_g_o_n_a_l_i_s_i_n_g, form of list comprehension is
provided (see separate manual section).
Note that list comprehensions do NOT remove duplicates from the result
list. To remove duplicates from a list, apply the standard function
`mkset'.
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