GHC Language

• >>GHC Syntax
• >>GHC Execution Model

>>GHC Syntax

H :- G1, G2, . . ., Gn | B1, B2, . . ., Bm.

• Above form is called a "clause"
• H is "head", H and G form "guard" and B is "body"
• H, G, B are predicates
  • ex.1: true
  • ex.2: copy(Xs, Ys)
• G, B are also called "goals"
• List expression:
  • [a|b] and [a, b] are syntactic sugars for cons(a, b) and cons(a, cons(b, nil)), respectively
• Symbols starting with a capital letter or '_' are variables
  • A variable namded '_' is an anonymous variable
  • Multiple anonymous variables in a clause are all unique

>>GHC Execution Model

• Execution of a GHC program is to replace goals
• Goals separated by ',' are executed concurrently
• If the pattern of a goal matches a head, the guard of the clause is tried
• Upon which variables may be instantiated only if they are not from calling side
• Trial is done concurrently
• Chooses just one clause whose guard has succeeded, and replaces the original goal by the body
• Body goals may instantiate variables from the calling side

GHC is a simple language, as execution models of languages are not usually described as short as above.