40 lines
951 B
Text
40 lines
951 B
Text
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fun map (f, l) =
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match (l) { N -> N | C (x, xs) -> C (f (x), map (f, xs)) }
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fun concat (l1, l2) =
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match (l1) { N -> l2 | C (x, xs) -> C (x, concat (xs, l2)) }
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fun flatten (l) =
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match (l) { N -> N | C (l, ls) -> concat (l, flatten (ls)) }
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fun return (x) = C (x, N)
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fun bind (x, f) = flatten (map (f, x))
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let fail = N
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fun some d = C (d, N)
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fun range (start, stop) =
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if (start >? stop) then { N } else { C (start, range (start + 1, stop)) }
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fun all_sums (start, stop, n) =
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bind (range (start, stop), \x ->
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bind (range (start, stop), \y ->
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if (x + y =? n) then { return (some (x, y, n)) } else { fail }
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))
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fun show_sum (x, y, n) =
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print_int (x);
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print_string (" + ");
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print_int (y);
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print_string (" = ");
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print_int (n);
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print_string ("\n")
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fun iter (f, l) =
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match (l) { N -> nothing | C (x, xs) -> f (x); iter (f, xs) }
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let test =
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iter (show_sum, flatten (all_sums (13, 73, 73)))
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