# 6. Function definition¶

## 6.1. Syntax to define function¶

Let’s write a function f to triple the argument.

 1 2 3 4 5 6 (* File: func_syntax.dats *) #include "share/atspre_staload.hats" fun f (x:int): int = 3 * x implement main0 () = println! (f 4) 
$patscc func_syntax.dats$ ./a.out
12


The function f is typed as following figure that explains the argument x is typed with int and the return value is also typed with int. The function f is typed with (int) -> int.

A function having two arguments can be written with following:

 1 2 3 4 5 6 (* File: two_arguments.dats *) #include "share/atspre_staload.hats" fun g (x:int, y:int): int = x * x + y * y - 4 implement main0 () = println! (g (3, 2)) 
$patscc two_arguments.dats$ ./a.out
9


The function g is typed with following figure, and it’s not curried form. However, currying is rarely used at ATS programming.

Note

Exercise: Write a function bmi that takes a meter tall and a kilogram weight then returns the body mass index (BMI). The BMI is calculated with dividing the kilogram weight by the square of the meter tall.

## 6.2. Typechecking¶

What happen if the function f takes a value typed double?

 1 2 3 4 5 6 7 8 9 (* File: f_takes_double.dats *) #include "share/atspre_staload.hats" fun f (x:int): int = 3 * x implement main0 () = { val r = f 4.0 val () = println! r } 
$patscc f_takes_double.dats /home/kiwamu/doc/ATS_Foundations/source/code/function_definition/f_takes_double.dats: 135(line=7, offs=14) -- 138(line=7, offs=17): error(3): the dynamic expression cannot be assigned the type [S2Eapp(S2Ecst(g0int_t0ype); S2Ecst(int_kind))]. /home/kiwamu/doc/ATS_Foundations/source/code/function_definition/f_takes_double.dats: 135(line=7, offs=14) -- 138(line=7, offs=17): error(3): mismatch of static terms (tyleq): The actual term is: S2Eapp(S2Ecst(g0float_t0ype); S2Ecst(double_kind)) The needed term is: S2Eapp(S2Ecst(g0int_t0ype); S2Ecst(int_kind)) patsopt(TRANS3): there are  errors in total. exit(ATS): uncaught exception: _2home_2kiwamu_2src_2ATS_2dPostiats_2src_2pats_error_2esats__FatalErrorExn(1025)  A compile error occurs at the code val r = f 4.0 while typechecking. It means the function f takes double type that not match type of f as following figure: You should create a new function f2 to maintain double type on the function f.  1 2 3 4 5 6 7 8 9 (* File: f_takes_double2.dats *) #include "share/atspre_staload.hats" fun f2 (x:double): double = 3 * x implement main0 () = { val r = f2 4.0 val () = println! r }  $ patscc f_takes_double2.dats
\$ ./a.out
12.000000


The code f_takes_double2.dats can be compiled. The function f2 is typed as following figure:

Note

Exercise: Write a function that takes a number of cranes and returns the total of their legs. (Crane has two legs.)

Note

Exercise: Write a function that takes a number of tortoises and returns the total of their legs. (Tortoise has four legs.)

Note

Exercise: Finally, write a function that takes the total of cranes and tortoises and the total of their legs, and return the total of the cranes.