I have a simple function: revFn (x:xs) = (revFn xs) . (x:) Of course GHCi correctly infers the type as: revFn :: [a] -> [a] -> c Adding the base case: revFn [] xs = xs Now gives an error; "Equations for 'revF' have different numbers of arguments" Of course this can be "fixed" by either adding the cancelled argument to the first clause, or converting the base case to only have one explicit argument, and a RHS of a lambda or identity function. But since the interpreter already correctly inferred that the first clause has two arguments (with only one explicit), why does it then ignore this and give an error when the second clause shows two explicit arguments? The types are all correct in either case - why require explicit arguments? Or, perhaps I am missing something simple?
You need to turn the second case into a lambda:
revFn [] = \xs -> xs
Or more succinctly,
revFn [] = id
On Mon, Nov 20, 2017 at 11:13 AM, Gregory Guthrie
I have a simple function:
revFn (x:xs) = (revFn xs) . (x:)
Of course GHCi correctly infers the type as: revFn :: [a] -> [a] -> c
Adding the base case:
revFn [] xs = xs
Now gives an error;
“Equations for ‘revF’ have different numbers of arguments”
Of course this can be “fixed” by either adding the cancelled argument to the first clause, or converting the base case to only have one explicit argument, and a RHS of a lambda or identity function.
But since the interpreter already correctly inferred that the first clause has two arguments (with only one explicit), why does it then ignore this and give an error when the second clause shows two explicit arguments? The types are all correct in either case – why require explicit arguments?
Or, perhaps I am missing something simple?
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On 2017-11-20 10:13 AM, Gregory Guthrie wrote:
But since the interpreter already correctly inferred that the first clause has two arguments (with only one explicit), why does it then ignore this and give an error when the second clause shows two explicit arguments? The types are all correct in either case – why require explicit arguments?
Actually, the first clause doesn't have two arguments. It's a single-argument function that returns a one-argument function. Strictly speaking, all multi-argument functions are one-argument functions that return other functions, but the compiler makes a distinction, perhaps to help guard against mistakes. So it's a requirement that all clauses of a function definition have the same number of explicit arguments.
On Mon, 2017-11-20 at 11:13 -0600, Gregory Guthrie wrote:
I have a simple function: revFn (x:xs) = (revFn xs) . (x:) Of course GHCi correctly infers the type as: revFn :: [a] -> [a] -> c Adding the base case: revFn [] xs = xs Now gives an error; “Equations for ‘revF’ have different numbers of arguments” Of course this can be “fixed” by either adding the cancelled argument to the first clause, or converting the base case to only have one explicit argument, and a RHS of a lambda or identity function. But since the interpreter already correctly inferred that the first clause has two arguments (with only one explicit), why does it then ignore this and give an error when the second clause shows two explicit arguments? The types are all correct in either case – why require explicit arguments? Or, perhaps I am missing something simple?
The semantics are based on Term Rewriting Systems. The number of arguments determines when the left side gets rewritten (evaluated) to the right side. This makes a difference in time and space properties. I think that is why Haskell (GHC) makes it explicit, and requires all parts of the function definition to have the same number of arguments. hope this helps, Arjen
On 2017-11-20 12:13 PM, Gregory Guthrie wrote:
I have a simple function:
revFn (x:xs) = (revFn xs) . (x:) [...]
revFn [] xs = xs
Now gives an error;
“Equations for ‘revF’ have different numbers of arguments”
Please see Haskell 2010 section 4.4.3.1 "Function bindings" and look for "and the number of patterns in each clause must be the same".
participants (5)
-
Albert Y. C. Lai -
Arjen -
Gregory Guthrie -
Neil Mayhew -
Zemyla