Question

Apologies for another question, I think I shall re-phrase my original question here as this is probably a pretty simple thing to do:

If i had a language which i wanted to create, like so:

A B <something> C

this would mean only an input like: A B …. C would be valid.

In contrast, B A ….. would not be valid.

Is there any easy way to define a constructor such as ‘B’ must always occur after ‘A’? I have only ever seen the A | B type of constructor ‘A or B’

Answer 1

Like so?

-- file Temp.hs
module Temp where
data C = C1 | C2 | C3
data B = B1 Int | B2 String | B3
data A a = MkA B a C

This compiles fine with ghc -c Temp.hs – you can’t use data statements in ghci, that’s one of the limitations of ghci.

So, here I’m defining three data types:

• C, which has three constructors which take no arguments: C1, C2, and C3. Each of these is a value of type C
• B, which has constructors
  • B1 which wraps a Int value, so B1 3 and B1 544 are values of type B
  • B2 which wraps a String value, so B2 "hello" and B2 "world" are values of type B
  • B3 is a simple argument-less constructor, so B3 is a value of type B.
• A a which is a polymorphic data type – for every type a, A a is also a data type. It has one constructor, MkA which takes
  • a value of type B
  • value of type a
  • and a value of type C

  and creates a value of type A a. So, for instance

  • MkA B3 True C1 is of type A Bool
  • MkA (B1 30) "Foo" C3 is of type A String