Haskell Cheat Sheet (Page 9 of 13) in pdf

scope of the

clause extends over all guards.

bind the value to. This facility is used below to

In contrast, the scope of a

expression is only

bind the head of the list in

for display, while also

and recalling the

deﬁnition of

from page

the current function clause and guard, if any.

binding the entire list to

in order to compute its

we can match on nested

values when

length:

present:

Declarations, Etc.

The following section details rules on function dec-

larations, list comprehensions, and other areas of

the language.

Pattern-matching also allows values to be assigned

Guards

Boolean functions can be used as

to variables. For example, this function determines

Function Deﬁnition

“guards” in function deﬁnitions along with pattern

if the string given is empty or not. If not, the value

matching. An example without pattern matching:

bound to

is converted to lower case:

Functions are deﬁned by declaring their name, any

arguments, and an equals sign:

All functions names must start with a lowercase let-

Note that

above is similer to

in that it will

ter or “ ”. It is a syntax error otherwise.

match anything; the only difference is that the

Notice

– it always evaluates to true and

value matched is also given a name.

can be used to specify a “default” branch.

Pattern Matching

Multiple “clauses” of a func-

Guards can be used with patterns. Here is a

tion can be deﬁned by “pattern-matching” on the

k Patterns

This (sometimes controversial)

function that determines if the ﬁrst character in a

values of arguments. Here, the the

function

pattern-matching facility makes it easy to match

string is upper or lower case:

has four separate cases:

certain kinds of numeric expressions.

The idea

is to deﬁne a base case (the “n” portion) with a

constant number for matching, and then to deﬁne

other matches (the “k” portion) as additives to the

base case. Here is a rather inefﬁcient way of testing

if a number is even or not:

Matching & Guard Order

Pattern-matching

proceeds in top to bottom order. Similarly, guard

expressions are tested from top to bottom. For ex-

ample, neither of these functions would be very in-

Note that the ‘ ’ character is a wildcard and

Argument Capture

Argument capture is useful

teresting:

matches any value.

for pattern-matching a value and using it, without

Pattern matching can extend to nested values.

declaring an extra variable. Use an ‘ ’ symbol in

Assuming this data declaration:

between the pattern to match and the variable to

c 2009 Justin Bailey.

Haskell Cheat Sheet Page 9

Related Articles

Related forms

Related Categories