This logic puzzle is adapted from Peter
Norvig's pytudes.
The key aspect of the *étude* is a neat description of
the problem that naturally leads to the solution. The code is
structured to resemble English statements as close as
possible.

We translate each of the 6 statements composing the problem
into MIT-Scheme
code. At the language level, note the use of
the `filter` procedure, an idiomatic Scheme sequence
operation
(see SICP,
Section '2.2.3 Sequences as Conventional Interfaces'). We use
it to replace Python iterables (e.g., list comprehensions),
too.

- Albert and Bernard just became friends with Cheryl, and
they want to know when her birthday is. Cheryl gave them a list of 10
possible dates:
May 15 May 16 May 19 June 17 June 18 July 14 July 16 August 14 August 15 August 17

- Cheryl then tells Albert and Bernard separately the month and the day of the birthday respectively.
- Albert: I don't know when Cheryl's birthday is, but I know that Bernard does not know too.
- Bernard: At first I don't know when Cheryl's birthday is, but I know now.
- Albert: Then I also know when Cheryl's birthday is.
- So when is Cheryl's birthday?

```
;;; 1. Cheryl gave them a list of 10 possible dates.
(define +dates+ '(("May" 15) ("May" 16) ("May" 19)
("June" 17) ("June" 18)
("July" 14) ("July" 16)
("August" 14) ("August" 15) ("August" 17)))
;; We'll also define accessor functions for the month and day of a
;; date:
(define (month date)
(car date))
(define (day date)
(cadr date))
(month '("May" 15))
#| "May" |#
(day '("May" 15))
#| 15 |#
;;; 2. Cheryl then tells Albert and Bernard separately the month and
;;; the day of the birthday respectively.
;; We can define the idea of telling, and while we're at it, the idea
;; of knowing a birth date.
(define (tell part)
"Cheryl tells a part of her birthdate to someone. Return a new list
of possible dates that match the part."
(define (contains-part? pair)
(member part pair))
(filter contains-part? +dates+))
(define (know possible-dates)
"A person knows the birthdate if they have exactly one possible
date."
(= 1 (length possible-dates)))
;; Note that we use a list of dates to represent someone's knowledge
;; of the possible birthdates, and that someone knows the birthdate
;; when they get down to only one possibility. For example: If Cheryl
;; tells Albert that her birthday is in May, he would have a list of
;; three possible birthdates:
(tell "May")
#| (("May" 15) ("May" 16) ("May" 19)) |#
;; And if she tells Bernard that her birthday is on the 15th, he would
;; end up with two possibilities:
(tell 15)
#| (("May" 15) ("August" 15)) |#
;; With two possibilities, Bernard does not know the birthdate:
(know (tell 15))
#| #f |#
;;; Overall strategy
;; When Cheryl tells Albert 'May' then he knows there are three
;; possibilities, but we (the puzzle solvers) don't, because we
;; don't know what Cheryl said. So what can we do? We will consider
;; all of the possible dates, one at a time. For example, first
;; consider 'May 15'. Cheryl tells Albert 'May' and Bernard
;; '15', giving them the lists of possible birthdates shown above.
;; We can then check whether statements 3 through 5 are true in this
;; scenario. If they are, then 'May 15' is a solution to the
;; puzzle. Repeat the process for each of the possible dates. If all
;; goes well, there should be exactly one solution.
;; Here is the main function, cheryls-birthday:
(define (cheryls-birthday)
(filter statement-3-to-5 +dates+))
(define (statement-3-to-5 date)
(and (statement3 date) (statement4 date) (statement5 date)))
;;; 3. Albert: I don't know when Cheryl's birthday is, but I know that
;;; Bernard does not know too.
;; The function `statement3` takes as input a possible birthdate and
;; returns true if Albert's statement is true for that birthdate. How
;; do we go from Albert's English statement to a Scheme procedure?
;; Let's paraphrase in a form that is closer to Scheme code:
;; Albert: After Cheryl told me the month of her birthdate, I didn't
;; know her birthday. I don't know which day Cheryl told Bernard,
;; but I know that for all of the possible dates, if Bernard is told
;; that day, he wouldn't know the birthdate.
;; That I can translate directly into code:
(define (statement3 date)
"Albert: I don't know when Cheryl's birthday is, but I know that
Bernard does not know too."
(define (not-know-day? d) ; Helper procedure
(not (know (tell (day d)))))
(let ((possible-dates (tell (month date)))) ; Statement
(and (not (know possible-dates))
(every not-know-day? possible-dates))))
;; We can try the function on a date:
(statement3 '("May" 15))
#| #f |#
(filter statement3 +dates+)
#| (("July" 14) ("July" 16) ("August" 14) ("August" 15) ("August" 17)) |#
;;; 4. Bernard: At first I don't know when Cheryl's birthday is, but I
;;; know now.
;; Again, a paraphrase:
;; Bernard: At first Cheryl told me the day, and I didn't know.
;; Then I considered just the dates for which Albert's statement 3
;; is true, and now I know.
(define (statement4 date)
"Bernard: At first I don't know when Cheryl's birthday is, but I
know now."
(let ((at-first (tell (day date))))
(and (not (know at-first))
(know (filter statement3 at-first)))))
;; Let's see which dates satisfy both statement 3 and statement 4:
(filter statement4 (filter statement3 +dates+))
#| (("July" 16) ("August" 15) ("August" 17)) |#
;; Wait a minute--I thought that Bernard knew?! Why are there three
;; possible dates remaining? Bernard does indeed know the birthdate,
;; because he knows something we don't know: the day. We won't know the
;; birthdate until after statement 5.
;;; Albert: Then I also know when Cheryl's birthday is.
;; Albert is saying that after hearing the month and Bernard's
;; statement 4, he now knows Cheryl's birthday:
(define (statement5 date)
"Albert: Then I also know when Cheryl's birthday is."
(know (filter statement4 (tell (month date)))))
;;; 6. So when is Cheryl's birthday?
;; Let's see:
(cheryls-birthday)
#| (("July" 16)) |#
;; Success! We have deduced that Cheryl's birthday is July 16. It is
;; now true that we know Cheryl's birthday:
(know (cheryls-birthday))
#| #t |#
```

MIT (Expat) License

- Original code and comments:
Copyright 2010-2017 Peter Norvig

- Scheme code:
Copyright 2018 Francesco Montanari

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

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