<s>
Parsec	B-Language
is	O
a	O
library	B-Library
for	O
writing	O
parsers	B-Language
in	O
Haskell	B-Language
.	O
</s>
<s>
It	O
is	O
based	O
on	O
higher-order	O
parser	B-Application
combinators	I-Application
,	O
so	O
a	O
complicated	O
parser	B-Language
can	O
be	O
made	O
out	O
of	O
many	O
smaller	O
ones	O
.	O
</s>
<s>
Erlang	B-Operating_System
,	O
</s>
<s>
Elixir	B-Language
,	O
</s>
<s>
F#	B-Operating_System
,	O
</s>
<s>
Java	B-Language
.	O
</s>
<s>
Because	O
a	O
parser	O
combinator-based	O
program	O
is	O
generally	O
slower	O
than	O
a	O
parser	O
generator-based	O
program	O
,	O
Parsec	B-Language
is	O
normally	O
used	O
for	O
small	O
domain-specific	B-Language
languages	I-Language
,	O
while	O
Happy	B-Language
is	O
used	O
for	O
compilers	O
such	O
as	O
GHC	B-Application
.	O
</s>
<s>
Other	O
Haskell	B-Language
parser	B-Application
combinator	I-Application
libraries	O
that	O
have	O
been	O
derived	O
from	O
Parsec	B-Language
include	O
Megaparsec	O
and	O
Attoparsec	O
.	O
</s>
<s>
Parsec	B-Language
is	O
free	B-Application
software	I-Application
released	O
under	O
the	O
BSD-3-Clause	O
license	O
.	O
</s>
<s>
Parsers	B-Language
written	O
in	O
Parsec	B-Language
start	O
with	O
simpler	O
parsers	B-Language
,	O
such	O
as	O
ones	O
that	O
recognize	O
certain	O
strings	O
,	O
and	O
combine	O
them	O
to	O
build	O
a	O
parser	B-Language
with	O
more	O
complicated	O
behavior	O
.	O
</s>
<s>
Parser	B-Application
combinator	I-Application
libraries	O
like	O
Parsec	B-Language
provide	O
utility	O
functions	O
to	O
run	O
the	O
parsers	B-Language
on	O
real	O
values	O
.	O
</s>
<s>
A	O
parser	B-Language
to	O
recognize	O
a	O
single	O
digit	O
from	O
a	O
string	O
can	O
be	O
split	O
into	O
two	O
functions	O
:	O
one	O
to	O
create	O
the	O
parser	B-Language
,	O
and	O
a	O
main	O
function	O
that	O
calls	O
one	O
of	O
these	O
utility	O
functions	O
(	O
parse	O
in	O
this	O
case	O
)	O
to	O
run	O
the	O
parser:We	O
define	O
a	O
Parser	B-Language
type	O
to	O
make	O
the	O
type	O
signature	O
of	O
parser	B-Language
easier	O
to	O
read	O
.	O
</s>
<s>
If	O
we	O
wanted	O
to	O
alter	O
this	O
program	O
,	O
say	O
to	O
read	O
either	O
the	O
string	O
"	O
hello	O
"	O
or	O
the	O
string	O
"	O
goodbye	O
"	O
,	O
we	O
could	O
use	O
the	O
operator	O
<|>	O
,	O
provided	O
by	O
the	O
Alternative	O
typeclass	O
,	O
to	O
combine	O
two	O
parsers	B-Language
into	O
a	O
single	O
parser	B-Language
that	O
tries	O
either	O
:	O
</s>
