How can I use Char value in example?

[masaomi-yamaguchi]

I am using oopsla2020-revision.

How can I give an example that includes Char value?
For example, I gave following json:

root@e85cf5f88368:/home/hoogle_plus# stack exec -- hplus --json='{"query": "a -> b -> (a,b)","inExamples": [{"inputs": ["'a'","'b'"],"output": "('a','b')"}]}'

Then, Hoogle+ returned following result:

RESULTS:{"outCandidates":[],"outDocs":[],"outError":"examples does not type check\nVariable not in scope: a\nVariable not in scope: b\nVariable not in scope: a\nVariable not in scope: b\n\n\nCallStack (from HasCallStack):\n  error, called at src/HooglePlus/Synthesize.hs:119:21 in HooglePlus-0.1.0.0-Ik8tTtBtUji4knXiVynJca:HooglePlus.Synthesize"}
hplus: examples does not type check
Variable not in scope: a
Variable not in scope: b
Variable not in scope: a
Variable not in scope: b


CallStack (from HasCallStack):
  error, called at src/HooglePlus/Synthesize.hs:119:21 in HooglePlus-0.1.0.0-Ik8tTtBtUji4knXiVynJca:HooglePlus.Synthesize
CallStack (from HasCallStack):
  error, called at src/HooglePlus/Synthesize.hs:125:10 in HooglePlus-0.1.0.0-Ik8tTtBtUji4knXiVynJca:HooglePlus.Synthesize
[error]: examples does not type check
[error]: Variable not in scope: a
[error]: Variable not in scope: b
[error]: Variable not in scope: a
[error]: Variable not in scope: b
[error]: 
[error]: 
[error]: CallStack (from HasCallStack):
[error]:   error, called at src/HooglePlus/Synthesize.hs:119:21 in HooglePlus-0.1.0.0-Ik8tTtBtUji4knXiVynJca:HooglePlus.Synthesize
root@e85cf5f88368:/home/hoogle_plus# 

Hoogle+ can handle Char-type, but I cannot find a way to use Char value in examples.
How can I use Char value in examples?