Difference between revisions of "Pattern Matching Assignment"
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===6) rev_string=== | ===6) rev_string=== | ||
− | Write a function <code>rev_string</code> that takes a string and returns the string that is the same characters in | + | Write a function <code>rev_string</code> that takes a <code>string</code> and returns the <code>string</code> that is the same characters in |
reverse order. Use ML’s o operator, the library function [https://smlfamily.github.io/Basis/list.html#SIG:LIST.rev:VAL rev] for reversing lists, and two library functions | reverse order. Use ML’s o operator, the library function [https://smlfamily.github.io/Basis/list.html#SIG:LIST.rev:VAL rev] for reversing lists, and two library functions | ||
in the [https://smlfamily.github.io/Basis/string.html String] module. (Browse [https://smlfamily.github.io/Basis/string.html the module documentation] to find the most useful functions.) | in the [https://smlfamily.github.io/Basis/string.html String] module. (Browse [https://smlfamily.github.io/Basis/string.html the module documentation] to find the most useful functions.) |
Revision as of 03:12, 8 October 2020
Contents
Hints and Gotchas
Instructions
All credit for this assignment goes to Prof. Grossman and his team at UW. UW or Coursera
Code To Implement
You will define several SML functions. Many will be very short because they will use other higher-order functions. You may use functions in ML’s library; the problems point you toward the useful functions and often require that you use them. The sample solution is about 120 lines, including the provided code, but not including the challenge problem. This assignment is probably more difficult than Homework 2 even though (perhaps because) many of the problems have 1-line answers.
String Functions
1) only_captials
Write a function only_capitals
that takes a string list
and returns a string list
that has only
the strings in the argument that start with an uppercase letter. Assume all strings have at least 1
character. Use List.filter, Char.isUpper, and String.sub to make a 1-2 line solution.
2) longest_string1
Write a function longest_string1
that takes a string list
and returns the longest string
in the
list. If the list is empty, return "". In the case of a tie, return the string closest to the beginning of the
list. Use foldl, String.size, and no recursion (other than the implementation of foldl is recursive).
3) longest_string2
Write a function longest_string2
that is exactly like longest_string1
except in the case of ties
it returns the string closest to the end of the list. Your solution should be almost an exact copy of
longest_string1. Still use foldl and String.size.
4) longest_string_helper, longest_string3, and longest_string4
Write functions longest_string_helper
, longest_string3
, and longest_string4
such that:
- longest_string3 has the same behavior as longest_string1 and longest_string4 has the same behavior as longest_string2.
- longest_string_helper has type (int * int -> bool) -> string list -> string (notice the currying). This function will look a lot like longest_string1 and longest_string2 but is more general because it takes a function as an argument.
- If longest_string_helper is passed a function that behaves like > (so it returns true exactly when its first argument is stricly greater than its second), then the function returned has the same behavior as longest_string1.
- longest_string3 and longest_string4 are defined with val-bindings and partial applications of longest_string_helper.
5) longest_capitalized
Write a function longest_capitalized
that takes a string list
and returns the longest string in
the list that begins with an uppercase letter, or "" if there are no such strings. Assume all strings
have at least 1 character. Use a val-binding and the ML library’s o operator for composing functions.
Resolve ties like in problem 2.
6) rev_string
Write a function rev_string
that takes a string
and returns the string
that is the same characters in
reverse order. Use ML’s o operator, the library function rev for reversing lists, and two library functions
in the String module. (Browse the module documentation to find the most useful functions.)
Utilities For Pattern Matching
The next two problems involve writing functions over lists that will be useful in later problems.
7) first_answer
Write a function first_answer of type (’a -> ’b option) -> ’a list -> ’b (notice the 2 arguments are curried). The first argument should be applied to elements of the second argument in order until the first time it returns SOME v for some v and then v is the result of the call to first_answer. If the first argument returns NONE for all list elements, then first_answer should raise the exception NoAnswer. Hints: Sample solution is 5 lines and does nothing fancy.
8) all_answers
Write a function all_answers of type (’a -> ’b list option) -> ’a list -> ’b list option (notice the 2 arguments are curried). The first argument should be applied to elements of the second argument. If it returns NONE for any element, then the result for all_answers is NONE. Else the calls to the first argument will have produced SOME lst1, SOME lst2, ... SOME lstn and the result of all_answers is SOME lst where lst is lst1, lst2, ..., lstn appended together (order doesn’t matter). Hints: The sample solution is 8 lines. It uses a helper function with an accumulator and uses @. Note all_answers f [] should evaluate to SOME [].
Pattern Matching
The remaining problems use these type definitions, which are inspired by the type definitions an ML implementation would use to implement pattern matching:
datatype pattern = Wildcard | Variable of string | UnitP | ConstP of int | TupleP of pattern list | ConstructorP of string * pattern datatype valu = Const of int | Unit | Tuple of valu list | Constructor of string * valu
Given valu v and pattern p, either p matches v or not. If it does, the match produces a list of string * valu pairs; order in the list does not matter. The rules for matching should be unsurprising:
- Wildcard matches everything and produces the empty list of bindings.
- Variable s matches any value v and produces the one-element list holding (s,v).
- UnitP matches only Unit and produces the empty list of bindings.
- ConstP 17 matches only Const 17 and produces the empty list of bindings (and similarly for other integers).
- TupleP ps matches a value of the form Tuple vs if ps and vs have the same length and for all i, the ith element of ps matches the ith element of vs. The list of bindings produced is all the lists from the nested pattern matches appended together.
- ConstructorP(s1,p) matches Constructor(s2,v) if s1 and s2 are the same string (you can compare them with =) and p matches v. The list of bindings produced is the list from the nested pattern match. We call the strings s1 and s2 the constructor name.
- Nothing else matches.
9)
10)
11)
12)
Challenge Problem
13) typecheck_patterns
Type Summary
val g = fn : (unit -> int) -> (string -> int) -> pattern -> int val only_capitals = fn : string list -> string list val longest_string1 = fn : string list -> string val longest_string2 = fn : string list -> string val longest_string_helper = fn : (int * int -> bool) -> string list -> string val longest_string3 = fn : string list -> string val longest_string4 = fn : string list -> string val longest_capitalized = fn : string list -> string val rev_string = fn : string -> string val first_answer = fn : (’a -> ’b option) -> ’a list -> ’b val all_answers = fn : (’a -> ’b list option) -> ’a list -> ’b list option val count_wildcards = fn : pattern -> int val count_wild_and_variable_lengths = fn : pattern -> int val count_some_var = fn : string * pattern -> int val check_pat = fn : pattern -> bool val match = fn : valu * pattern -> (string * valu) list option val first_match = fn : valu -> pattern list -> (string * valu) list option
val typecheck_patterns = fn : (string * string * typ) list * pattern list -> typ option
Test
file: | unit_test_hw3.sml | |
source folder: | src/test/sml/hw3 |
Test your functions. The unit tests you have been provided are intentionally minimal.
Pledge, Acknowledgments, Citations
file: | lab-3-card-game-date-pledge-acknowledgments-citations.txt |
More info about the Honor Pledge