CODEWITHSUNDEEP
listdeclarationpredicatemercury
dogwood
dogwood

Reputation: 363

How to declare a pred for a predicate that imports or outputs lists?

Looking for help declaring a pred for a predicate that imports or outputs lists. I tried :- pred name(list::in, integer::out) is multi. and compiler error message says that list/0 isn't recognized. Checked library module list and see that I should write something like ...(list(T)::in . . . ), but didn't fully understand what to do.

Upvotes: 2

Views: 102

Answers (2)

Paul Bone
Paul Bone

Reputation: 822

List is a parametric type, parametric types take one or more parameters. In the case of list the parameter says what this is a list of. You may have a list of numbers, a list of strings, a list of pumpkins or a list of lists of numbers (any valid type). So, if I create a function such as:

:- func max(list(int)) = int.

This function takes a list of ints and returns an int (the maximum number found in the list).

So, what's with list(T)? A token beginning with a capital letter is a variable, even in types, It can stand for any other type (usually). So "list(T)" means a list of anything, such as a list of numbers or strings. The next predicate is polymorphic, it works for different types depending on the actual values of it's type variable.

:- pred first(list(T)::in, T::out) is semidet.

A list of anything can be passed as the first item in the list will be returned, if there is one. If this is used with a list of strings "list(string)" then T will be substituted (during compilation) with "string".

The reference for this part of Mercury's type system is here. http://www.mercurylang.org/information/doc-release/mercury_ref/Discriminated-unions.html#Discriminated-unions

Upvotes: 3

dogwood
dogwood

Reputation: 363

I have tried :- pred name(...list(T),....) is ... and the compiler is no longer reporting errors related to my pred declaration. Don't yet know that the results will be correct, but one step closer.

Upvotes: 0

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