[xsmith-dev] XSmith questions

[Sorawee Porncharoenwase]

Hi everyone,

I just started using XSmith. I don’t think I grok the framework yet, so I
would really appreciate it if anyone could help me!

1) While I understand that dynamically typed language should specify
type-info to generate programs that don’t cause runtime type mismatch, I
find it weird that types are very integral to the framework. For example,
binder-info seems to presuppose that declaration AST nodes will have type
information in it.

On one hand, that’s strictly speaking not the case for many languages.

On the other hand, I can understand that the specified grammar here doesn’t
need to correspond to the actual language grammar. So let’s just add the
type field into the AST node. But this raises another question: how do I
know which other AST nodes will require additional type information as well?

2) As far as I can see, all declaration nodes have types to be that of the
expression it contains. Again, I find this weird since declaration in most
languages is a statement, not an expression. I tried fixing this by
modifying the “Another Small Example with Variables” example to:

(add-property
 arith
 type-info
 [Definition [no-return-type (λ (n t) (hash 'Expression
(fresh-type-variable)))]]
 [LetStar [(fresh-type-variable)
           (λ (n t) (hash 'definitions (λ (cn) no-return-type)
                          'sideEs (λ (cn) (fresh-type-variable))
                          'Expression t))]]
 ...)

where no-return-type is from xsmith/canned-components. The generation
errors with with:

Exception:
unify!: subtype-unify!: can't unify these types: #<type-variable
(#<range:#<no-return-type>-#<no-return-type>>)> and #<int>

In most examples that I saw, declarations are hoisted at the very top of
function / block / program. I think the no-return-type is particularly
going to be a problem when I want to allow declarations and statements to
interleave, since statements will be assigned a unique type (like
no-return-type), while declarations can’t do the same.

3) How do I debug when XSmith simply hangs and doesn’t output anything? It
looks like the problems I encountered are with types, since if I relax the
constraints, it can generate programs without any problem (though the
generated programs are incorrect, as expected).

4) In xsmith/canned-components, there are several predefined types, but the
module only exports constructors, not accessors. What would be the best way
to extract information from it? unify!?

Thanks!
Sorawee
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