Introduction to Computational Linguistics

3. Welcome To The Typed Universe 5
isn’t there (the most frequent mistake).
3 Welcome To The Typed Universe
In OCaML, every expression is typed. There are certain basic types, but types can
also be created. The type is named by a string. The following are inbuilt types
(the list is not complete, you find a description in the user manual):
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character
string
integer
boolean
float
char
string
int
bool
float
There are conventions to communicate a token of a given type. Characters must be
enclosed in single quotes. ’a’, for example refers to the character a. Strings must
be enclosed in double quotes: "mail" denotes the string mail. The distinction
matters: "a" is a string, containing the single letter a. Although identical in
appearance to the character a, they must be kept distinct. The next type, integer
is typed in as such, for example 10763. Notice that "10763" is a string, not
a number! Booleans have two values, typed in as follows: true and false.
Finally, float is the type of floating point numbers. They are distinct from integers,
although they may have identical values. For example, 1.0 is a number of type
float, 1 is a number of type int. Type in 1.0 = 1;; and OCaML will respond
with an error message:
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# 1.0 = 1;;
This expression has type int but is here used with type
float
And it will underline the offending item. It parses expressions from left to right
and stops at the first point where things do not match. First it sees a number of
type float and expects that to the right of the equation sign there also is a number
of type float. When it reads the integer it complains. For things can only be equal
via = if they have identical type.
The typing is very strict. Whenever something is declared to be of certain type,
it can only be used with operations that apply to things of that type. Moreover,