This is documentation for Mathematica 5, which was
based on an earlier version of the Wolfram Language.

 2.3.4 Specifying Types of Expression in Patterns You can tell a lot about what "type" of expression something is by looking at its head. Thus, for example, an integer has head Integer, while a list has head List. In a pattern, _h and x_h represent expressions that are constrained to have head h. Thus, for example, _Integer represents any integer, while _List represents any list. Patterns for objects with specified heads. This replaces just those elements that are integers. In[1]:= {a, 4, 5, b} /. x_Integer -> p[x] Out[1]= You can think of making an assignment for f[x_Integer] as like defining a function f that must take an argument of "type" Integer. This defines a value for the function gamma when its argument is an integer. In[2]:= gamma[n_Integer] := (n - 1)! The definition applies only when the argument of gamma is an integer. In[3]:= gamma[4] + gamma[x] Out[3]= The object 4. has head Real, so the definition does not apply. In[4]:= gamma[4.] Out[4]= This defines values for expressions with integer exponents. In[5]:= d[x_^n_Integer] := n x^(n-1) The definition is used only when the exponent is an integer. In[6]:= d[x^4] + d[(a+b)^3] + d[x^(1/2)] Out[6]=