basic, first draft Julia implementation

A fairly direct port from Python

1 files changed, 128 insertions, 0 deletions

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+
+# A partial Scheme implementation in Julia
+
+#   null -> nothing
+#   pair -> Pair type
+#   boolean (true/false) -> true/false
+#   number -> Number
+#   identifier/symbol -> symbol
+
+is_atom(x) = x == () || !(x == nothing ||
+                          typeof(x) <: Tuple ||
+                          typeof(x) <: Array ||
+                          typeof(x) <: Dict)
+is_null(x) = x == ()
+is_zero(x) = x == 0
+is_eq(a, b) = a == b
+is_number(x) = typeof(x) <: Number
+is_boolean(x) = x == true || x == false
+is_builtin(x) = x in (:lambda, :quote, :cond, :else, :cons, :car, :cdr,
+                      :isnull, :iseq, :isatom, :iszero, :isnumber,
+                      :+, :-, :*, :/, +, -, *, /)
+
+identity_action(x, ctx) = x
+lookup_action(x, ctx) = ctx[x]
+quote_action(x, ctx) = length(x) == 2 ? x[2] : throw(ErrorException())
+
+function cond_action(x, ctx)
+    for line in x[2:end]
+        if line[1] == :else
+            return meaning(line[2], ctx)
+        elseif meaning(line[1], ctx)
+            return meaning(line[2], ctx)
+        else
+            # "unspecified"
+            return nothing
+        end
+    end
+end
+
+function lambda_action(x, ctx)
+    return (:procedure, x[2], x[3:end], copy(ctx))
+end
+
+function apply_action(x, ctx)
+    if is_builtin(x[1])
+        action = x[1]
+        args = [meaning(y, ctx) for y in x[2:end]]
+        if action == :cons
+            return tuple(args[1], args[2])
+        elseif action == :car
+            return args[1][1]
+        elseif action == :cdr
+            return args[1][2]
+        elseif action == :isnull
+            return is_null(args[1])
+        elseif action == :iseq
+            return is_eq(args[1], args[2])
+        elseif action == :isatom
+            return is_atom(args[1])
+        elseif action == :iszero
+            return is_zero(args[1])
+        elseif action == :isnumber
+            return is_number(args[1])
+        elseif action == :+ || action == +
+            return args[1] + args[2]
+        elseif action == :- || action == -
+            return args[1] - args[2]
+        elseif action == :* || action == *
+            return args[1] * args[2]
+        elseif action == :/ || action == /
+            return args[1] / args[2]
+        else
+            throw(ErrorException("Unexpected builtin: $(x[0])"))
+        end
+    elseif typeof(x[1]) <: Tuple
+        proc = meaning(x[1], ctx)
+        if proc[1] != :procedure
+            throw(ErrorException("Not applicable: $(str(proc))"))
+        end
+        variables = proc[2]
+        body = proc[3]
+        closure = copy(proc[4])
+        args = [meaning(y, ctx) for y in x[2:end]]
+        for i in 1:length(variables)
+            closure[variables[i]] = args[i]
+        end
+        ret = nothing
+        for expr in body
+            ret = meaning(expr, closure)
+        end
+        return ret
+    else
+        throw(ErrorException("Unexpected... thing...: $(x[1])"))
+    end
+end
+
+function meaning(x, ctx)
+    action = nothing
+    if is_atom(x)
+        if is_number(x) || is_boolean(x) || is_builtin(x)
+            return identity_action(x, ctx)
+        elseif typeof(x) <: Symbol
+            return lookup_action(x, ctx)
+        end
+    elseif typeof(x) <: Tuple
+        if x[1] == :quote
+            return quote_action(x, ctx)
+        elseif x[1] == :cond
+            return cond_action(x, ctx)
+        elseif x[1] == :lambda
+            return lambda_action(x, ctx)
+        elseif typeof(x[1]) <: Tuple
+            return apply_action(x, ctx)
+        else   # some other identifier, either builtin or not
+            return apply_action(x, ctx)
+        end
+    end
+    throw(ErrorException("Unexpected expression: $x"))
+end
+
+function value(ast)
+    return meaning(ast, Dict())
+end
+
+function test_minimal_scheme()
+    @assert 6 == value( ((:lambda, (:x, ), (+, 1, :x)), 5) )
+    value( (:car, (:quote, (1, 2, 3, 4))) )
+end