92 lines
2.6 KiB
Rust
92 lines
2.6 KiB
Rust
use crate::parser::Datum;
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use crate::parser::Datum::*;
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use crate::primitives::*;
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use std::mem;
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fn is_true(exp: &Datum) -> bool {
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match exp {
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Boolean(b) => *b,
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Number(_) => true,
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List(_) => true,
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_ => panic!("IS-TRUE -- unknown-truth-value {:?}", exp),
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}
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}
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fn lookup_variable_value(exp: &Datum) -> Datum {
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if let Symbol(string) = exp {
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match string.as_str() {
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"+" => Procedure(add),
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"*" => Procedure(mul),
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"-" => Procedure(sub),
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_ => panic!("LOOKUP-VARIABLE-VALUE -- unbound-variable {:?}", string),
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}
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} else {
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panic!("LOOKUP-VARIABLE-VALUE -- not-a-symbol {:?}", exp)
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}
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}
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fn application(exp: Datum) -> Datum {
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if let List(vector) = exp {
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if vector.len() == 0 {
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panic!("APPLICATION -- no-procedure")
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}
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let mut args: Vec<Datum> = vector.into_iter().map(interpret).collect();
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if let Procedure(proc) = mem::take(&mut args[0]) {
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// FIXME: call procedures with args only
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proc(args)
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} else {
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panic!("APPLICATION -- not-aplicable {:?}", args[0])
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}
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} else {
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panic!("APPLICATION -- cannot-apply {:?}", exp)
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}
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}
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fn has_tag(exp: &Datum, tag: &str) -> bool {
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if let List(l) = exp {
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if l.len() == 0 {
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return false;
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}
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if let Symbol(s) = &l[0] {
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if s == tag {
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return true;
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}
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}
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}
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false
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}
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fn interpret_if(mut args: Vec<Datum>) -> Datum {
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let predicate = mem::take(&mut args[1]);
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let alternative = mem::take(&mut args[2]);
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if is_true(&predicate) {
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interpret(alternative)
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} else {
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if args.len() == 4 {
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let consequent = mem::take(&mut args[3]);
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interpret(consequent)
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} else {
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Unspecified
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}
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}
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}
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pub fn interpret(exp: Datum) -> Datum {
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match exp {
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Boolean(_) => exp,
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Number(_) => exp,
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Symbol(_) => lookup_variable_value(&exp),
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List(v) if has_tag(&exp, "if") => interpret_if(v),
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List(_) if has_tag(&exp, "set!") => panic!("assignment-not-supported"),
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List(_) if has_tag(&exp, "define") => panic!("define-not-supported"),
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List(_) if has_tag(&exp, "cond") => panic!("cond-not-supported"),
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List(_) if has_tag(&exp, "let") => panic!("let-not-supported"),
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List(_) if has_tag(&exp, "begin") => panic!("begin-not-supported"),
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List(_) => application(exp),
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// TODO: quoted
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_ => panic!("unknown-expression {:?}", exp),
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}
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}
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