Runtime evaluation: Difference between revisions

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=={{header|Slate}}==

{{incorrect|Slate|It does not discuss passing in or returning values, or the environment the expression is evaluated in., and it does not show taking in an arbitrary program as from user input.}}

Here is some code in Slate that uses run-time evaluation.

In Slate, programs are represented as Syntax Node trees, with methods defined on the various syntactic types. The backtick syntax provides a convenient quoting mechanism, and as objects, they have convenient methods defined for evaluation or evaluation within a specific environment:

`(4 + 5) evaluate.

w@(Gtk Workspace traits) evaluateSelected [

`(4 + 5) evaluateIn: prototypes.

| selection parser result |

</lang>

You can also explicitly invoke the Parser on a String, to convert it into syntactic objects:

selection: w selectedTextOrCurrentLine.

parser: (Syntax Parser newOn: selection reader).

(Syntax Parser newOn: '4 + 5') upToEnd do: [| :each | print: each evaluate]

[ parser isAtEnd ] whileFalse: [

result: (parser next evaluateIn: w namespace)

].

result

].

</lang>

You can construct a program using externally-specified values using <tt>`unquote</tt> within a quoted expression:

define: #x -> 4.

`(x `unquote + 5) evaluate.

</slate>

Or you can obviously construct a string:

define: #x -> 4.

(Syntax Parser newOn: x printString ; ' + 5')

</lang>

The <tt>evaluate</tt> method also takes into consideration the current lexical scope, unless another environment is specified. The following returns 10, no matter what binding <tt>x</tt> has in the local namespace:

[| x | x: 5. `(x `unquote + 5) evaluate] do.

</lang>

Slate can sandbox via constructing a fresh namespace and evaluating within it, but this mechanism is not strongly secure yet.

=={{header|Smalltalk}}==