A format checker is a strategy that checks the well-formedness of an term.
Format checkers can check more properties than can just be described using
signatures. For example, checking that a term is normalized in a certain way.
Format checkers can be defined easily using RecursivePattern.
Consider the following signature of a simple expression language:
signature
sorts Exp
constructors
Var : String -> Exp
Add : Exp * Exp -> Exp
Mul : Exp * Exp -> Exp
The following format checking strategy =Exp= checks that
a term is a well-formed =Exp= term.
strategies
Exp = Var(is-string) + Add(Exp, Exp) + Mul(Exp, Exp)
It is also possible to check more strict properties such
as being an additive expression (if that is a term), i.e.,
an expression consisting of additions on top and multiplications
within. No addition should occur as a sub-term of a multiplication.
The following strategy definition is a more concise way of
expressing this property:
AdditiveExp =
rec a(Add(a, a) + rec m(Mul(m, m) + Var(is-string)))
Note that the use of the =rec= RecursionOperator makes the definition
more concise (without it an additional strategy definition would be needed).
-- Main.EelcoVisser - 28 Nov 2001

The tool =format-check= (available from StrategoXT 0.10) checks if an ATerm is part of the language defined by an abstract syntax definition in [[RegularTreeGrammar][RTG]].
-- Main.MartinBravenboer - 29 Jun 2004