The goal of Homework #3 is to implement the interpreter of MRFWAE, which is the extended version of FWAE with Multi-arugments and Records.
1 Functions with Multiple Arguments
Implement MRFWAE to support any number of arguments to a function (including zero), and any number of arguments (including zero) in a function application:
<MRFWAE ::= <number
| {+ <MRFWAE <MRFWAE}
| {- <MRFWAE <MRFWAE}
| {with {<id <MRFWAE} <MRFWAE}
| <id
| {<MRFWAE <MRFWAE*}
| {fun {<id*} <MRFWAE}
For parsing, any alphanumeric string other than with, or fun can be an identifier. At run-time, a new error is now possible: function application with the wrong number of arguments. Your interpreter should detect the mismatch and report an error that includes the words "wrong arity":
test(run("{{fun {x y} {+ x y}} 1 2}"), "3") test(run("{{fun {} {+ 3 4}}}"), "7")
testExc(run("{{fun {x y} {+ x y}} 1}"), "wrong arity")
2 Adding Records
Extend your interpreter to support the construction of records with named fields, and to support field selection from a record:
<MRFWAE ::= ...
| {record {<id <MRFWAE}*}
| {access <MRFWAE <id}
Adding records means that the language now has three kinds of values: numbers, functions, and records. At run-time, an error may occur because a record is misused as a number, a number is supplied to access, or a record supplied to access does not have the named field. Your error message for the last case should include the words "no such field", otherwise you can make up your own error messages (or just let primitive error checking handle problems, such as trying to add a record to a number).
test(run("{access {record {x 1} {y 2}} x}"), "1")
testExc(run("{access {record {x 1} {y 2}} z}"), "no such field") testExc(run("{record {x {access {record {y 1}} z}}}"), "no such field")
1
Since records are now values, the result of the interpreter can be a record, not only a number or a function. For homework purposes, we don’t want to nail down the representation of a record result, because there are many choices. The examples below therefore use run, which is a wrapper on your interpreter that just returns a number string if it produces a number, and it returns the string "function" if it produces a function value, but it returns the string "record" if it produces a record value:
test(run("42"), "42")
test(run("{fun {x} x}"), "function") test(run("{record {x 1}}"), "record")
