`if` in evaluator.
Chris Pressey
1 year, 8 months ago
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* import statements
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* let_star statements
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* `if`
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* cond
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* more thorough mangling
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* compile to file named on command line
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### Forms
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* `if`
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* `=`
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* `map`
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* `=` -- this only works on numbers
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* `map` -- this is in Scheme
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* `fold`
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* `error`
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* `assert`
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* `error` -- this is in Chicken
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* `assert` -- this is in Chicken too
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* Comparison operators on strings -- what does Scheme do for this?
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else
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local result = eval_expr(test, env)
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if is_a(result, Boolean) and result.value() then
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return eval_expr(b.tail().head(), env)
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return eval_expr(b.tail().head(), env)
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end
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end
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branch = branch.tail()
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end
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error("No else in cond")
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elseif head.text() == "if" then
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local sexp = ast.tail()
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local result = eval_expr(sexp.head(), env)
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if not is_a(result, Boolean) or result.value() then
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return eval_expr(sexp.tail().head(), env)
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else
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return eval_expr(sexp.tail().tail().head(), env)
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end
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elseif head.text() == "lambda" then
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local formals = ast.tail().head()
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local body = ast.tail().tail().head()
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Decision-making
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---------------
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`if`.
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(if (equal? (quote a) (quote a)) (quote true) (quote false))
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===> true
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(if (equal? (quote a) (quote b)) (quote true) (quote false))
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===> false
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All non-booleans are considered true. This at least coincides with
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what Chicken tells me.
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(if (list 1) (quote true) (quote false))
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===> true
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(if (list) (quote true) (quote false))
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===> true
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(if 1 (quote true) (quote false))
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===> true
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(if 0 (quote true) (quote false))
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===> true
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(if "hey" (quote true) (quote false))
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===> true
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(if "" (quote true) (quote false))
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===> true
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`cond` works.
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