Unexpected undefined

[emilaxelsson]

> eval $ case Indexed 0 (\i -> err "sdf") Empty :: Matrix Index of Indexed l ixf Empty -> (ixf 0) 
*** Exception: Prelude.undefined

Expected result

*** Exception: sdf

Due to constant folding?

[emilaxelsson]

It seems to be the calls to getLength and getIx in thawVector (coming from the sugaring of err) that cause the problems. A simpler test case is:

> drawAST $ \a -> a + getLength (err "aaa" :: Data [Index])
*** Exception: Assert failed: aaa

Disabling constant folding removes the problem.

We need to make sure not to constant fold undefined values. Currently, every closed expression with a result type in Type gets constant folded. But I don't think it's enough to turn off constant folding for the constructs in Constructs.Error because

• Expressions like

  1 + getLength (err "aaa" :: Data [Index])
  

  would still be folded.

• We have other sources of undefined values:

  > drawAST $ \a -> a + getIx (value [] :: Data [Index]) 1
  *** Exception: List.genericIndex: index too large.
  

I'm not sure exactly why the error gets propagated in these examples, but possible reasons are

• Size propagation (sizeOf is strict)
• Optimizations matching on specific literals

I think the solution is to lift our semantic domain to e.g. Maybe a. That way, we don't have to use undefined in expressions. This change would probably affect Syntactic (since we don't want the users to see the lifted domain).