I don't understand mueval's design anyway here:
do the interpreter options mean that these are automatically on,
or just that the source text will be allowed to switch then on?
(I'd prefer the latter.)
I'm not clear on what you're doing there (I don't recognize your snippet), but extensions are enabled as options:
-X EXTENSION --extension=EXTENSION Pass additional flags enabling extensions just like you would to ghc. Example: -XViewPatterns
So for example:
$ mueval -e "let f (id -> x) = x in f 1" -XViewPatterns 1 $ mueval -e "let f (id -> x) = x in f 1" -XMultiParamTypeClasses
Illegal view pattern: id -> x Use -XViewPatterns to enable view patterns