2015 003d STRING

signature STRING

We propose three new functions for the STRING signature.

This page is part of proposal 2015-003.

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Synopsis

val rev           : string -> string
val implodeRev    : char list -> string

val concatWithMap : string -> ('a -> string) -> 'a list -> string

Description

• rev s

returns the reverse of `s`.

• implodeRev chrs

is equivalent to the expression `implode (List.rev chrs)`.

• concatWithMap sep f items

returns a string that is constructed by converting the items to strings using the function `f` and then joining them using the separator string `sep`. For example, `concatWithMap "," Int.toString [1, 2, 3]` evaluates to the string `"1,2,3"`.

Discussion

We might also want to add a concatRev operation that is equivalent to

concat o List.rev

for the same reason that implodeRev is useful.

I am not sure what the best order for the arguments to concatWithMap is. I decided to put the separator first, because I could imagine wanting to format lists of various different element types using the same separator. Furthermore, since it follows the order of the pattern that it replaces; i.e.,

concatWith sep (List.map f items)

On the other hand, the Basis Library convention for higher-order combinators has been to put the function argument first.

Rationale

The rev function provides an efficient way to reverse a string, which is an occasionally useful operation. The implodeRev functions is useful for when a list of characters has been constructed, but is in the wrong order. The concatWithMap function captures an extremely common pattern where one must first map a to-string conversion over a list and the concatenate the result with a separator. Combining the map and concatenation in a single function provides convinence to the programmer and is amenable to a more efficient implementation.