how recursive data structure work in haskell

Question

This is a recursive data structure in Haskell, how does it work?

data Expression
     = Var Variable
     | Num Integer
     | Plus  Expression Expression
     | Minus Expression Expression
     | Times Expression Expression
     | Div   Expression Expression

data Variable = A | B

Minus (Plus (Var A)(Var B)) (VarB) : how does this expression work?

Have you read a tutorial? e.g. The Gentle Intro and Learn You a Haskell both have fine sections that should help you refine this question to something a bit more... focused. — Daniel Wagner
– Daniel Wagner, Commented Aug 21, 2012 at 10:32
Actually, it shouldn't work as it stands now. The type is called Expersssion (3 s's) and the recursive uses are to Experssion (2 s's) :-) — yatima2975
– yatima2975, Commented Aug 21, 2012 at 10:58

Will Ness · Accepted Answer · 2012-08-21 12:16:29Z

1

by recursively matching its sub-parts against the types of the corresponding data constructors:

Minus    (Plus (Var A) (Var B))    (Var B)
Minus ::      Expression   ->     Expression  -> Expression

Plus           (Var A)             (Var B)
Plus  ::      Expression   ->     Expression  -> Expression

Var               A
Var   ::       Variable                       -> Expression

Var               B
Var   ::       Variable                       -> Expression

A     ::       Variable

B     ::       Variable

edited Aug 21, 2012 at 12:16

answered Aug 21, 2012 at 10:41

Will Ness

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