The Subject to Reason About

345    “Two added to one—if that could but be done,”
346        It said, “with one’s fingers and thumbs!”
347    Recollecting with tears how, in earlier years,
348        It had taken no pains with its sums.

349    “The thing can be done,” said the Butcher, “I think.
350        The thing must be done, I am sure.
351    The thing shall be done! Bring me paper and ink,
352        The best there is time to procure.”

353    The Beaver brought paper,portfolio, pens,
354        And ink in unfailing supplies:
355    While strange creepy creatures came out of their dens,
356        And watched them with wondering eyes.

357    So engrossed was the Butcher, he heeded them not,
358        As he wrote with a pen in each hand,
359    And explained all the while in a popular style
360        Which the Beaver could well understand.

357    So engrossed was the Butcher, he heeded them not,
358        As he wrote with a pen in each hand,
359    And explained all the while in a popular style
360        Which the Beaver could well understand.

361    “Taking Three as the subject to reason about—
362        A convenient number to state—
363    We add Seven, and Ten, and then multiply out
364        By One Thousand diminished by Eight.

365    “The result we proceed to divide, as you see,
366        By Nine Hundred and Ninety Two:
367    Then subtract Seventeen, and the answer must be
368        Exactly and perfectly true.

369    “The method employed I would gladly explain,
370        While I have it so clear in my head,
371    If I had but the time and you had but the brain—
372        But much yet remains to be said.

 
GHCi, version 7.10.3: http://www.haskell.org/ghc/ :? for help
Prelude> :set prompt "λ> "
λ> :set +t
λ> let y x = (x + 7 + 10) * (1000 - 8) / 992 - 17
y :: Fractional a => a -> a
λ> y 3
3.0
it :: Fractional a => a
λ> y 42
42.0
it :: Fractional a => a
λ>

That works with any subject to reason about.

 
related blog post