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73 lines
1.5 KiB
Plaintext
73 lines
1.5 KiB
Plaintext
:- module expr.
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:- interface.
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:- import_module char, int, list.
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:- type token
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---> ('+')
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; ('-')
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; ('*')
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; ('/')
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; num(int)
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; ('(')
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; (')')
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; eof
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.
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:- parse(exprn/1, token, eof, xx, in, out).
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:- pred scan(list(char), list(token)).
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:- mode scan(in, out) is det.
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:- implementation.
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:- import_module string, require.
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:- rule exprn(int).
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exprn(Num) ---> exprn(A), [+], term(B), { Num = A + B }.
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exprn(Num) ---> exprn(A), [-], term(B), { Num = A - B }.
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exprn(Num) ---> term(Num).
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:- rule term(int).
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term(Num) ---> term(A), [*], factor(B), { Num = A * B }.
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term(Num) ---> term(A), [/], factor(B), { Num = A // B }.
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term(Num) ---> factor(Num).
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:- rule factor(int).
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factor(Num) ---> ['('], exprn(Num), [')'].
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factor(Num) ---> [num(Num)].
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scan(Chars, Toks) :-
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scan(Chars, [], Toks0),
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list__reverse(Toks0, Toks).
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:- pred scan(list(char), list(token), list(token)).
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:- mode scan(in, in, out) is det.
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scan([], Toks, [eof|Toks]).
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scan([C|Cs], Toks0, Toks) :-
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( char__is_whitespace(C) ->
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scan(Cs, Toks0, Toks)
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; char__is_digit(C) ->
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takewhile(char__is_digit, [C|Cs], Digits, Rest),
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string__from_char_list(Digits, NumStr),
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Num = string__det_to_int(NumStr),
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scan(Rest, [num(Num)|Toks0], Toks)
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; C = ('+') ->
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scan(Cs, ['+'|Toks0], Toks)
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; C = ('-') ->
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scan(Cs, ['-'|Toks0], Toks)
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; C = ('*') ->
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scan(Cs, ['*'|Toks0], Toks)
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; C = ('/') ->
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scan(Cs, ['/'|Toks0], Toks)
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; C = ('(') ->
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scan(Cs, ['('|Toks0], Toks)
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; C = (')') ->
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scan(Cs, [')'|Toks0], Toks)
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;
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error("expr: syntax error in input")
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).
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