Program design should be taught with a comprehensible guideline and
appropriate tool support. While Felleisen et al.'s program design recipe serves as a good guideline
for novice learners, no existing tool provides sufficient support for
step-by-step design. We propose Mio, an environment for designing programs based on the design recipe. In Mio, the programmer uses blocks to express design artifacts, such as
examples of input and output data. The system checks the consistency of the design, gives feedback to the
programmer, and produces a half-completed program for use in steps after
designing. A preliminary experiment in the classroom showed its ability to make
program design easier for novices, and to encourage programmers to follow
the design recipe. In this paper, we demonstrate the core features of Mio, report the results
of the experiment, and discuss our plans for extensions.