CODEWITHSUNDEEP
sml
ahpto
ahpto

Reputation: 21

Simple question re: Passing a parameterized datatype in SML

I just spent an embarrassing amount of time figuring out that if you're passing a parameterized datatype into a higher-order function in SML, it needs to be in brackets (); so, for example:

fun f1 p = f2 p will work when called like this (for example): f1(Datatype(parameter)) but will not work if called like f1 Datatype(parameter). I'm sure there's a very simple reason why, but I'm not quite clear. Is it something like, the datatype and parameter are "seen" as 2 things by the function if not in brackets? Thanks!

Upvotes: 0

Views: 56

Answers (1)

Chris
Chris

Reputation: 36536

It's important to realize how functions work in SML. Functions take a single argument, and return a single value. This is very easy to understand but it's very often practically necessary for a function to take more than one value as input. There are two ways of achieving this:

Tuples

A function can take one value that contains multiple values in the form of a tuple. This is very common in SML. Consider for instance:

fun add (x, y) = x + y

Here (x, y) is a tuple inferred to be composed of two ints.

Currying

A function takes one argument and returns one value. But functions are values in SML, so a function can return a function.

fun add x = fn y => x + y

Or just:

fun add x y = x + y

This is common in OCaml, but less common in SML.

Function Application

Function application in SML takes the form of functionName argument. When a tuple is involved, it looks like: functionName (arg1, arg2). But the space can be elided: functionName(arg1, arg2).

Even when tuples are not involved, we can put parentheses around any value. So calling a function with a single argument can look like: functionName argument, functionName (argument), or functionName(argument).

Your Question

f1(Datatype(parameter))

This parses the way you expect.

f1 Datatype(parameter)

This parses as f1 Datatype parameter, which is a curried function f1 applied to the arguments Datatype and parameter.

Upvotes: 2

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